diff --git a/src/main/java/algorithms/sprint0/SlidingAverage.java b/src/main/java/algorithms/sprint0/SlidingAverage.java
index 868506e..189fb1f 100644
--- a/src/main/java/algorithms/sprint0/SlidingAverage.java
+++ b/src/main/java/algorithms/sprint0/SlidingAverage.java
@@ -28,7 +28,7 @@ static List<Double> movingAverage(int n, List<Integer> arr, int windowSize) {
         }
         result.add(sum / (double) windowSize);
         for (int i = windowSize; i < minSize; i++) {
-            sum += arr.get(i) - arr.get(i - windowSize);
+            sum += (long) arr.get(i) - arr.get(i - windowSize);
             result.add(sum / (double) windowSize);
         }
         return result;
diff --git a/src/main/java/algorithms/sprint1/Distances.java b/src/main/java/algorithms/sprint1/Distances.java
index fbb674e..a7041fd 100644
--- a/src/main/java/algorithms/sprint1/Distances.java
+++ b/src/main/java/algorithms/sprint1/Distances.java
@@ -3,8 +3,6 @@
 import java.io.BufferedInputStream;
 import java.io.BufferedOutputStream;
 import java.io.EOFException;
-import java.io.FileInputStream;
-import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
@@ -12,11 +10,13 @@
 
 // https://contest.yandex.ru/contest/22450/run-report/157289807/
 public class Distances {
+    private static final int MAX_INPUT_SIZE = 100_000;
 
     // -------------------- SOLUTION --------------------
     static int[] solve(int[] a) {
         int n = a.length;
         int[] dist = new int[n];
+        if (n == 0) return dist;
 
         int lastZero = -n;
         for (int i = 0; i < n; i++) {
@@ -113,18 +113,20 @@ void writeInt(int x) throws IOException {
         void flush() throws IOException {
             out.write(buf, 0, p);
             p = 0;
+            out.flush();
         }
     }
 
     // -------------------- INPUT / OUTPUT --------------------
-    static void run() {
-
-        try (InputStream input = new BufferedInputStream(new FileInputStream("input.txt"));
-             OutputStream output = new BufferedOutputStream(new FileOutputStream("output.txt"))) {
-            FastIn in = new FastIn(input);
-            FastOut out = new FastOut(output);
+    static void run(InputStream input, OutputStream output) {
+        try {
+            FastIn in = new FastIn(new BufferedInputStream(input));
+            FastOut out = new FastOut(new BufferedOutputStream(output));
 
             int n = in.nextInt();
+            if (n < 1 || n > MAX_INPUT_SIZE) {
+                throw new IllegalArgumentException("Input size must be between 1 and " + MAX_INPUT_SIZE);
+            }
             int[] a = new int[n];
             for (int i = 0; i < n; i++) a[i] = in.nextInt();
 
@@ -141,6 +143,10 @@ static void run() {
         }
     }
 
+    static void run() {
+        run(System.in, System.out);
+    }
+
     // -------------------- TESTS --------------------
     static void assertEq(int[] exp, int[] act, String name) {
         if (!Arrays.equals(exp, act)) {
@@ -149,6 +155,7 @@ static void assertEq(int[] exp, int[] act, String name) {
     }
 
     static void test() {
+        assertEq(new int[]{}, solve(new int[]{}), "empty");
         assertEq(new int[]{0, 1, 2, 1, 0}, solve(new int[]{0, 1, 4, 9, 0}), "sample1");
         assertEq(new int[]{0, 1, 2, 3, 4, 5}, solve(new int[]{0, 7, 9, 4, 8, 20}), "sample2");
         assertEq(new int[]{0}, solve(new int[]{0}), "n=1");
diff --git a/src/main/java/algorithms/sprint1/Solution2.java b/src/main/java/algorithms/sprint1/Solution2.java
index 66bb225..acab909 100644
--- a/src/main/java/algorithms/sprint1/Solution2.java
+++ b/src/main/java/algorithms/sprint1/Solution2.java
@@ -5,10 +5,13 @@
 import java.io.BufferedReader;
 import java.io.IOException;
 import java.io.InputStreamReader;
+import java.io.PrintWriter;
 import java.nio.charset.StandardCharsets;
 import java.util.StringTokenizer;
 
 public class Solution2 {
+    private static final int MAX_LINE_COUNT = 1_000_000;
+
     public static void solution(Node<String> head) {
         StringBuilder output = new StringBuilder();
         Node<String> current = head;
@@ -35,16 +38,47 @@ static void test() {
     }
 
     public static void main(String[] args) throws IOException {
-        StringBuilder outputBuffer = new StringBuilder();
         BufferedReader reader = new BufferedReader(new InputStreamReader(System.in, StandardCharsets.UTF_8));
-        int lineCount = parseInt(reader.readLine());
+        int lineCount = readLineCount(reader);
+        PrintWriter writer = new PrintWriter(System.out, false, StandardCharsets.UTF_8);
         for (int i = 0; i < lineCount; ++i) {
-            StringTokenizer tokenizer = new StringTokenizer(reader.readLine());
-            int firstValue = parseInt(tokenizer.nextToken());
-            int secondValue = parseInt(tokenizer.nextToken());
-            int result = firstValue + secondValue;
-            outputBuffer.append(result).append("\n");
+            StringTokenizer tokenizer = new StringTokenizer(readInputLine(reader, i + 1));
+            int firstValue = parseInt(nextToken(tokenizer, i + 1));
+            int secondValue = parseInt(nextToken(tokenizer, i + 1));
+            if (tokenizer.hasMoreTokens()) {
+                throw new IllegalArgumentException("Line " + (i + 1) + " must contain exactly two integers");
+            }
+            int result = Math.addExact(firstValue, secondValue);
+            writer.println(result);
+        }
+        writer.println();
+        writer.flush();
+    }
+
+    private static int readLineCount(BufferedReader reader) throws IOException {
+        String countLine = reader.readLine();
+        if (countLine == null) {
+            throw new IllegalArgumentException("Missing line count");
+        }
+        int lineCount = parseInt(countLine);
+        if (lineCount < 0 || lineCount > MAX_LINE_COUNT) {
+            throw new IllegalArgumentException("Line count must be between 0 and " + MAX_LINE_COUNT);
+        }
+        return lineCount;
+    }
+
+    private static String readInputLine(BufferedReader reader, int lineNumber) throws IOException {
+        String inputLine = reader.readLine();
+        if (inputLine == null) {
+            throw new IllegalArgumentException("Missing input line " + lineNumber);
+        }
+        return inputLine;
+    }
+
+    private static String nextToken(StringTokenizer tokenizer, int lineNumber) {
+        if (!tokenizer.hasMoreTokens()) {
+            throw new IllegalArgumentException("Line " + lineNumber + " must contain exactly two integers");
         }
-        System.out.println(outputBuffer);
+        return tokenizer.nextToken();
     }
 }
diff --git a/src/main/java/algorithms/sprint2/Calculator.java b/src/main/java/algorithms/sprint2/Calculator.java
index 5f1da48..0a5ad1c 100644
--- a/src/main/java/algorithms/sprint2/Calculator.java
+++ b/src/main/java/algorithms/sprint2/Calculator.java
@@ -45,9 +45,16 @@ private static int eval(FastIn in) throws IOException {
             if (t.length() == 1) {
                 char op = t.charAt(0);
                 if (op == '+' || op == '-' || op == '*' || op == '/') {
+                    if (st.size() < 2) {
+                        throw new IllegalArgumentException("Operator requires two operands: " + op);
+                    }
                     int b = st.pop();
                     int a = st.pop();
 
+                    if (op == '/' && b == 0) {
+                        throw new IllegalArgumentException("Division by zero");
+                    }
+
                     int r;
                     if (op == '+') {
                         r = a + b;
@@ -64,9 +71,16 @@ private static int eval(FastIn in) throws IOException {
                 }
             }
 
-            st.push(parseInt(t));
+            try {
+                st.push(parseInt(t));
+            } catch (NumberFormatException exception) {
+                throw new IllegalArgumentException("Invalid token: " + t, exception);
+            }
         }
 
+        if (st.size() != 1) {
+            throw new IllegalArgumentException("Expression must produce exactly one result");
+        }
         return st.peek();
     }
 
@@ -74,7 +88,13 @@ private static void run() throws Exception {
         FastIn in = new FastIn(System.in);
         FastOut out = new FastOut(System.out);
 
-        int ans = eval(in);
+        final int ans;
+        try {
+            ans = eval(in);
+        } catch (IllegalArgumentException exception) {
+            System.err.println("Invalid expression: " + exception.getMessage());
+            return;
+        }
 
         out.writeInt(ans);
         out.writeByte('\n');
diff --git a/src/main/java/algorithms/sprint2/Deque.java b/src/main/java/algorithms/sprint2/Deque.java
index c3f3141..408d102 100644
--- a/src/main/java/algorithms/sprint2/Deque.java
+++ b/src/main/java/algorithms/sprint2/Deque.java
@@ -68,11 +68,13 @@ static final class RingDeque {
         }
 
         private int next(int i) {
-            return (i+1) % cap;
+            i++;
+            return i == cap ? 0 : i;
         }
 
         private int prev(int i) {
-            return (i-1+cap) % cap;
+            i--;
+            return i == -1 ? cap - 1 : i;
         }
 
         boolean isEmpty() {
diff --git a/src/main/java/algorithms/sprint3/FastSort.java b/src/main/java/algorithms/sprint3/FastSort.java
index b5a4de2..da71ce5 100644
--- a/src/main/java/algorithms/sprint3/FastSort.java
+++ b/src/main/java/algorithms/sprint3/FastSort.java
@@ -8,6 +8,9 @@
 
 public class FastSort {
 
+    private static final int MAX_PARTICIPANTS = 100_000;
+    private static final int MAX_LOGIN_LENGTH = 256;
+
     /*
     Принцип работы алгоритма:
     1) Считываем n участников в массив.
@@ -140,24 +143,43 @@ private int read() throws IOException {
             return buffer[ptr++];
         }
 
-        int nextInt() throws IOException {
-            int c;
-            do {
-                c = read();
-                if (c == -1) {
-                    throw new EOFException();
-                }
-            } while (c <= ' ');
-
+        int nextInt(String fieldName, int minValue, int maxValue) throws IOException {
+            int c = nextNonWhitespace();
             int value = 0;
+
             while (c > ' ') {
-                value = value * 10 + c - '0';
+                if (c < '0' || c > '9') {
+                    throw new IOException("Invalid integer token for " + fieldName);
+                }
+                int digit = c - '0';
+                if (value > (maxValue - digit) / 10) {
+                    throw new IOException("Integer token for " + fieldName + " exceeds " + maxValue);
+                }
+                value = value * 10 + digit;
                 c = read();
             }
+
+            if (value < minValue) {
+                throw new IOException("Integer token for " + fieldName + " is below " + minValue);
+            }
             return value;
         }
 
-        String next() throws IOException {
+        String next(int maxLength) throws IOException {
+            int c = nextNonWhitespace();
+            StringBuilder sb = new StringBuilder(Math.min(maxLength, 16));
+
+            while (c > ' ') {
+                if (sb.length() == maxLength) {
+                    throw new IOException("Token length exceeds " + maxLength);
+                }
+                sb.append((char) c);
+                c = read();
+            }
+            return sb.toString();
+        }
+
+        private int nextNonWhitespace() throws IOException {
             int c;
             do {
                 c = read();
@@ -165,26 +187,20 @@ String next() throws IOException {
                     throw new EOFException();
                 }
             } while (c <= ' ');
-
-            StringBuilder sb = new StringBuilder();
-            while (c > ' ') {
-                sb.append((char) c);
-                c = read();
-            }
-            return sb.toString();
+            return c;
         }
     }
 
     private static void run() throws Exception {
         FastIn in = new FastIn(System.in);
 
-        int n = in.nextInt(); // n — количество участников
+        int n = in.nextInt("participant count", 0, MAX_PARTICIPANTS); // n — количество участников
         Participant[] participants = new Participant[n];
 
         for (int i = 0; i < n; i++) {
-            String login = in.next();
-            int solved = in.nextInt();
-            int penalty = in.nextInt();
+            String login = in.next(MAX_LOGIN_LENGTH);
+            int solved = in.nextInt("solved", 0, Integer.MAX_VALUE);
+            int penalty = in.nextInt("penalty", 0, Integer.MAX_VALUE);
             participants[i] = new Participant(login, solved, penalty);
         }
 
diff --git a/src/main/java/algorithms/sprint5/Solution.java b/src/main/java/algorithms/sprint5/Solution.java
index 82ccb6c..e2b03e1 100644
--- a/src/main/java/algorithms/sprint5/Solution.java
+++ b/src/main/java/algorithms/sprint5/Solution.java
@@ -26,7 +26,7 @@ public class Solution {
      *
      * Почему алгоритм корректен:
      * 1) По свойству BST искомый ключ может находиться только
-     *    в одном из двух поддеревьев, поэтому рекурсивный спуск
+     *    в одном из двух поддеревьев, поэтому спуск
      *    идёт ровно по нужному пути.
      *
      * 2) Если у удаляемой вершины не более одного ребёнка,
@@ -46,43 +46,49 @@ public class Solution {
      * Временная сложность: O(h), где h — высота дерева,
      * что в худшем случае даёт O(n), где n — число узлов в дереве.
      *
-     * Дополнительная пространственная сложность: O(h) из-за стека рекурсии,
-     * что в худшем случае даёт O(n).
+     * Дополнительная пространственная сложность: O(1), поскольку обход
+     * выполняется итеративно.
      */
 
     public static Node remove(Node root, int key) {
-        if (root == null) {
-            return null;
+        Node parent = null;
+        Node current = root;
+
+        while (current != null && current.getValue() != key) {
+            parent = current;
+            current = key < current.getValue() ? current.getLeft() : current.getRight();
         }
 
-        if (key < root.getValue()) {
-            root.setLeft(remove(root.getLeft(), key));
+        if (current == null) {
             return root;
         }
 
-        if (key > root.getValue()) {
-            root.setRight(remove(root.getRight(), key));
+        if (current.getLeft() != null && current.getRight() != null) {
+            Node predecessorParent = current;
+            Node predecessor = current.getLeft();
+            while (predecessor.getRight() != null) {
+                predecessorParent = predecessor;
+                predecessor = predecessor.getRight();
+            }
+
+            current.setValue(predecessor.getValue());
+            if (predecessorParent == current) {
+                predecessorParent.setLeft(predecessor.getLeft());
+            } else {
+                predecessorParent.setRight(predecessor.getLeft());
+            }
             return root;
         }
 
-        if (root.getLeft() == null) {
-            return root.getRight();
+        Node replacement = current.getLeft() != null ? current.getLeft() : current.getRight();
+        if (parent == null) {
+            return replacement;
         }
-
-        if (root.getRight() == null) {
-            return root.getLeft();
+        if (parent.getLeft() == current) {
+            parent.setLeft(replacement);
+        } else {
+            parent.setRight(replacement);
         }
-
-        Node predecessor = findMax(root.getLeft());
-        root.setValue(predecessor.getValue());
-        root.setLeft(remove(root.getLeft(), predecessor.getValue()));
         return root;
     }
-
-    private static Node findMax(Node node) {
-        while (node.getRight() != null) {
-            node = node.getRight();
-        }
-        return node;
-    }
 }
diff --git a/src/main/java/coderun/AverageElement.java b/src/main/java/coderun/AverageElement.java
index 461cce6..99c8dc6 100644
--- a/src/main/java/coderun/AverageElement.java
+++ b/src/main/java/coderun/AverageElement.java
@@ -3,7 +3,6 @@
 
 import static common.SafeParse.parseInt;
 
-import java.io.BufferedReader;
 import java.io.BufferedWriter;
 import java.io.IOException;
 import java.io.InputStreamReader;
@@ -11,12 +10,13 @@
 import java.io.Reader;
 import java.io.Writer;
 import java.nio.charset.StandardCharsets;
-import java.util.Arrays;
 
 
 
 public class AverageElement {
 
+    private static final int MAX_INPUT_CHARACTERS = 4096;
+
     /**
      * Для чтения входных данных необходимо получить их
      * из стандартного потока ввода (System.in).
@@ -58,26 +58,52 @@ public static void main(String[] args) throws IOException {
     }
 
     static void solve(Reader input, Writer output) throws IOException {
-        BufferedReader reader = new BufferedReader(input);
         BufferedWriter writer = new BufferedWriter(output);
-        String line = reader.readLine();
-        if (line == null) {
+        String line = readBoundedLine(input);
+        if (line == null || line.isBlank()) {
             return;
         }
         String[] parts = line.trim().split("\\s+");
 
-        writer.write(String.valueOf(average(parts)));
+        try {
+            writer.write(String.valueOf(average(parts)));
+        } catch (IllegalArgumentException ignored) {
+            return;
+        }
         writer.flush();
     }
 
     static Integer average(String[] list) {
-        return Arrays
-                .stream(list)
-                .sequential()
-                .map(AverageElement::parseInteger)
-                .sorted()
-                .toList()
-                .get(1);
+        if (list.length < 2) {
+            throw new IllegalArgumentException("At least two integers are required");
+        }
+
+        int smallest = Integer.MAX_VALUE;
+        int secondSmallest = Integer.MAX_VALUE;
+        for (String value : list) {
+            int number = parseInteger(value);
+            if (number < smallest) {
+                secondSmallest = smallest;
+                smallest = number;
+            } else if (number < secondSmallest) {
+                secondSmallest = number;
+            }
+        }
+        return secondSmallest;
+    }
+
+    private static String readBoundedLine(Reader input) throws IOException {
+        StringBuilder line = new StringBuilder();
+        for (int character = input.read(); character != -1 && character != '\n'; character = input.read()) {
+            if (character == '\r') {
+                break;
+            }
+            if (line.length() == MAX_INPUT_CHARACTERS) {
+                return null;
+            }
+            line.append((char) character);
+        }
+        return line.isEmpty() ? null : line.toString();
     }
 
     private static Integer parseInteger(String value) {
diff --git a/src/main/java/interview/MaximumSequenceWithOneZero.java b/src/main/java/interview/MaximumSequenceWithOneZero.java
index 55a3596..17decf2 100644
--- a/src/main/java/interview/MaximumSequenceWithOneZero.java
+++ b/src/main/java/interview/MaximumSequenceWithOneZero.java
@@ -1,7 +1,6 @@
 package interview;
 
 
-import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
 
@@ -16,24 +15,20 @@ public static int maximumSequenceWithOneZero(List<Integer> b) {
         if (b.size() == 1 && b.get(0) == 1) return 1;
         if (b.size() == 1 && b.get(0) == 0) return 0;
 
-        List<Integer> bytes = new ArrayList<>(b);
         int maxCount = 0;
-        int count = 0;
-        if (bytes.get(0) == 1) count++;
-
-        for (int i = 1; i < bytes.size(); i++) {
-            if (bytes.get(i) == 1 && bytes.get(i - 1) == 1) {
-                count++;
-            }
-            if (bytes.get(i) == 1 && bytes.get(i - 1) == 0) {
-                count++;
-            }
-            if (bytes.get(i) == 0 && bytes.get(i - 1) == 0) {
-                maxCount = Math.max(count, maxCount);
-                count = 0;
+        int previousCount = 0;
+        int currentCount = 0;
+
+        for (Integer value : b) {
+            if (value == 1) {
+                currentCount++;
+                maxCount = Math.max(maxCount, previousCount + currentCount);
+            } else {
+                previousCount = currentCount;
+                currentCount = 0;
             }
         }
-        return Math.max(count, maxCount);
+        return maxCount;
     }
 
 
diff --git a/src/main/java/kyu3/RailFenceCipher.java b/src/main/java/kyu3/RailFenceCipher.java
index 4c5c22f..6a8ff0f 100644
--- a/src/main/java/kyu3/RailFenceCipher.java
+++ b/src/main/java/kyu3/RailFenceCipher.java
@@ -1,7 +1,6 @@
 package kyu3;
 
 
-import java.util.LinkedList;
 import java.util.Map;
 import java.util.TreeMap;
 
@@ -11,6 +10,7 @@ public class RailFenceCipher {
     //3 https://www.codewars.com/kata/58c5577d61aefcf3ff000081/train/java
 
     static String encode(String s, int n) {
+        validateRailCount(n);
         // Distributes characters across rails using a periodic index that walks
         // up and down between boundary rails, then concatenates each rail content.
         Map<Integer, StringBuilder> map = new TreeMap<>();
@@ -37,57 +37,41 @@ static String encode(String s, int n) {
     }
 
     static String decode(String s, int n) {
+        validateRailCount(n);
         // First computes the exact size of each rail, splits ciphertext into
         // contiguous rail segments, then reconstructs plaintext by replaying the
         // same rail traversal cycle.
+        if (s.isEmpty() || n >= s.length()) {
+            return s;
+        }
+
         int[] size = new int[n];
         Counter counter = new Counter(n - 1);
         for (int i = 0; i < s.length(); i++) {
             int tik = counter.tik();
             size[tik]++;
         }
-        Map<Integer, StringBuilder> map = new TreeMap<>();
-        StringBuilder result = new StringBuilder();
+        int[] position = new int[n];
+        int start = 0;
         for (int i = 0; i < n; i++) {
-            StringBuilder stringBuilder = new StringBuilder();
-            if (i == 0) {
-                stringBuilder.append(s, 0, size[0]);
-            } else {
-                int start = sumArr(size, i);
-                int end = start + size[i];
-                stringBuilder.append(s, start, end);
-            }
-            map.put(i, stringBuilder);
-        }
-
-        Map<Integer, LinkedList<String>> map2 = new TreeMap<>();
-
-        for (Map.Entry<Integer, StringBuilder> e : map.entrySet()
-        ) {
-            LinkedList<String> strings = new LinkedList<>();
-            for (char c : e.getValue().toString().toCharArray()) {
-                strings.add(String.valueOf(c));
-            }
-            map2.put(e.getKey(), strings);
+            position[i] = start;
+            start += size[i];
         }
 
+        StringBuilder result = new StringBuilder(s.length());
         Counter counterRes = new Counter(n - 1);
         for (int i = 0; i < s.length(); i++) {
             int tik = counterRes.tik();
-            result.append(map2.get(tik).poll());
+            result.append(s.charAt(position[tik]++));
         }
 
         return result.toString();
     }
 
-    private static int sumArr(int[] arr, int i) {
-        // Sum widths of all previous rails to determine the start offset
-        // of the current rail segment.
-        int result = 0;
-        for (int j = 0; j < i; j++) {
-            result += arr[j];
+    private static void validateRailCount(int railCount) {
+        if (railCount < 2) {
+            throw new IllegalArgumentException("Rail count must be at least 2");
         }
-        return result;
     }
 
     /**
diff --git a/src/main/java/kyu5/PigLatin.java b/src/main/java/kyu5/PigLatin.java
index 4527a4a..32b5bdd 100644
--- a/src/main/java/kyu5/PigLatin.java
+++ b/src/main/java/kyu5/PigLatin.java
@@ -21,7 +21,9 @@ public static String pigIt(String str) {
         String[] words = str.split("\\s");
         for (String w : words
         ) {
-            if (!w.matches("\\W")) {
+            if (w.isEmpty()) {
+                continue;
+            } else if (!w.matches("\\W")) {
                 stringBuilder.append(w.substring(1)).append(w.charAt(0)).append(AY);
             } else {
                 stringBuilder.append(w).append(" ");
diff --git a/src/main/java/kyu6/CamelCase.java b/src/main/java/kyu6/CamelCase.java
index 1093287..c14043b 100644
--- a/src/main/java/kyu6/CamelCase.java
+++ b/src/main/java/kyu6/CamelCase.java
@@ -29,9 +29,10 @@ public static String fromCamelCase(String input) {
     }
 
     public static String toCamelCase(String str) {
+        if (str == null || str.isEmpty()) return "";
         return Arrays.stream(str.split(" "))
                 .filter(s -> s.length() > 0)
-                .map(s -> s.replaceFirst(s.substring(0, 1), s.substring(0, 1).toUpperCase(Locale.ROOT)))
+                .map(s -> s.substring(0, 1).toUpperCase(Locale.ROOT) + s.substring(1))
                 .collect(Collectors.joining()).trim();
     }
 
diff --git a/src/main/java/kyu6/EncryptThis.java b/src/main/java/kyu6/EncryptThis.java
index 452f4a0..e9fa91a 100644
--- a/src/main/java/kyu6/EncryptThis.java
+++ b/src/main/java/kyu6/EncryptThis.java
@@ -7,9 +7,9 @@ public class EncryptThis {
     //6 https://www.codewars.com/kata/5848565e273af816fb000449/train/java
 
     public static String encryptThis(String text) {
-        if (text == null || text.isEmpty()) return "";
+        if (text == null || text.isBlank()) return "";
         StringBuilder stringBuilder = new StringBuilder();
-        String[] split = text.trim().split("\\s");
+        String[] split = text.trim().split("\\s+");
         for (String s : split) {
             String res = "";
             if (s.length() == 1) {
diff --git a/src/main/java/kyu6/NumberInExpandedForm.java b/src/main/java/kyu6/NumberInExpandedForm.java
index e64151c..bb35662 100644
--- a/src/main/java/kyu6/NumberInExpandedForm.java
+++ b/src/main/java/kyu6/NumberInExpandedForm.java
@@ -8,6 +8,9 @@ public class NumberInExpandedForm {
     //6 https://www.codewars.com/kata/5842df8ccbd22792a4000245/train/java
 
     public static String expandedForm(int num) {
+        if (num == 0) {
+            return "0";
+        }
         StringBuilder result = new StringBuilder();
         char[] chars = String.valueOf(num).toCharArray();
         for (int i = 0; i < chars.length; i++) {
diff --git a/src/main/java/kyu6/TribonacciSequence.java b/src/main/java/kyu6/TribonacciSequence.java
index a8749d1..2438955 100644
--- a/src/main/java/kyu6/TribonacciSequence.java
+++ b/src/main/java/kyu6/TribonacciSequence.java
@@ -5,6 +5,7 @@
 public class TribonacciSequence {
 
     private static final int SIGNATURE_SIZE = 3;
+    private static final int MAX_SEQUENCE_LENGTH = 1_000_000;
 
     //6 https://www.codewars.com/kata/556deca17c58da83c00002db
 
@@ -12,8 +13,10 @@ public static double[] tribonacci(double[] s, int n) {
         if (s == null || s.length != SIGNATURE_SIZE) {
             throw new IllegalArgumentException("signature must contain exactly three values");
         }
-        if (n < 0) {
-            throw new IllegalArgumentException("sequence length must not be negative");
+        if (n < 0 || n > MAX_SEQUENCE_LENGTH) {
+            throw new IllegalArgumentException(
+                    "sequence length must be between 0 and " + MAX_SEQUENCE_LENGTH
+            );
         }
         double[] result = new double[n];
         int signatureLength = Math.min(SIGNATURE_SIZE, n);
diff --git a/src/main/java/kyu7/Maskify.java b/src/main/java/kyu7/Maskify.java
index 42ddda5..34bccb8 100644
--- a/src/main/java/kyu7/Maskify.java
+++ b/src/main/java/kyu7/Maskify.java
@@ -7,7 +7,7 @@ public class Maskify {
     //7 https://www.codewars.com/kata/5412509bd436bd33920011bc/train/java
 
     public static String maskify(String str) {
-        if (str.length() < 5) return str;
+        if (str == null || str.length() < 5) return str;
         return "#".repeat(str.length() - 4) + str.substring(str.length() - 4);
     }
 
diff --git a/src/main/java/other/RecursionMax.java b/src/main/java/other/RecursionMax.java
index 65022bf..29bab74 100644
--- a/src/main/java/other/RecursionMax.java
+++ b/src/main/java/other/RecursionMax.java
@@ -7,20 +7,7 @@
 public class RecursionMax {
 
     public static int recursionMax(List<Integer> array) {
-        if (array == null) {
-            return -1;
-        }
-        if (array.isEmpty()) {
-            return -1;
-        }
-        return recursionMax(array, 0, Integer.MIN_VALUE);
-    }
-
-    private static int recursionMax(List<Integer> array, int index, int currentMax) {
-        if (index == array.size()) {
-            return currentMax;
-        }
-        return recursionMax(array, index + 1, Math.max(currentMax, array.get(index)));
+        return max(array);
     }
 
     public static int max(List<Integer> array) {
diff --git a/src/test/java/algorithms/AlgorithmCliTest.java b/src/test/java/algorithms/AlgorithmCliTest.java
index 0da7388..fbe72a5 100644
--- a/src/test/java/algorithms/AlgorithmCliTest.java
+++ b/src/test/java/algorithms/AlgorithmCliTest.java
@@ -1,6 +1,7 @@
 package algorithms;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.params.provider.Arguments.arguments;
 
 import java.io.ByteArrayInputStream;
@@ -10,14 +11,13 @@
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.nio.charset.StandardCharsets;
-import java.nio.file.Files;
-import java.nio.file.Path;
 import java.util.stream.Stream;
 import org.junit.jupiter.api.Tag;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
+import org.junit.jupiter.params.provider.ValueSource;
 
 @Tag("integration")
 class AlgorithmCliTest {
@@ -34,20 +34,25 @@ void stdioEntryPointProducesExpectedOutput(
     }
 
     @Test
-    void distancesRunReadsAndWritesContestFiles() throws Exception {
-        Path input = Path.of("input.txt");
-        Path output = Path.of("output.txt");
-        Files.writeString(input, "5%n0 1 4 9 0%n".formatted(), StandardCharsets.UTF_8);
-        Files.deleteIfExists(output);
+    void solution2RejectsMissingInputLine() {
+        assertThrows(IllegalArgumentException.class, () -> invokeWithStdio(
+                "algorithms.sprint1.Solution2",
+                "main",
+                "2%n1 2%n".formatted()));
+    }
 
-        try {
-            invokeNoArg("algorithms.sprint1.Distances", "run");
+    @Test
+    void solution2RejectsOversizedLineCount() {
+        assertThrows(IllegalArgumentException.class, () -> invokeWithStdio(
+                "algorithms.sprint1.Solution2",
+                "main",
+                "1000001%n".formatted()));
+    }
 
-            assertEquals("0 1 2 1 0\n", normalizeLines(Files.readString(output, StandardCharsets.UTF_8)));
-        } finally {
-            Files.deleteIfExists(input);
-            Files.deleteIfExists(output);
-        }
+    @ParameterizedTest
+    @ValueSource(strings = {"+", "", "1 0 /", "abc", "1 2"})
+    void calculatorRejectsMalformedExpressionsWithoutThrowing(String input) throws Exception {
+        assertEquals("", invokeWithStdio("algorithms.sprint2.Calculator", "run", input));
     }
 
     private static Stream<Arguments> stdioCases() {
@@ -61,6 +66,9 @@ private static Stream<Arguments> stdioCases() {
                 arguments("algorithms.sprint1.SleightOfHand", "run",
                         "3%n1231%n2..2%n2..2%n2..2%n".formatted(),
                         "2%n".formatted()),
+                arguments("algorithms.sprint1.Distances", "run",
+                        "5%n0 1 4 9 0%n".formatted(),
+                        "0 1 2 1 0%n".formatted()),
                 arguments("algorithms.sprint1.Solution2", "main",
                         "2%n1 2%n3 4%n".formatted(),
                         "3%n7%n%n".formatted()),
@@ -154,10 +162,6 @@ private static String invokeWithStdio(String className, String methodName, Strin
         }
     }
 
-    private static void invokeNoArg(String className, String methodName) throws Exception {
-        invoke(className, methodName);
-    }
-
     private static void invoke(String className, String methodName) throws Exception {
         Method method = "main".equals(methodName)
                 ? Class.forName(className).getDeclaredMethod(methodName, String[].class)
diff --git a/src/test/java/algorithms/sprint0/SlidingAverageTest.java b/src/test/java/algorithms/sprint0/SlidingAverageTest.java
index 7042b1f..5dbcd44 100644
--- a/src/test/java/algorithms/sprint0/SlidingAverageTest.java
+++ b/src/test/java/algorithms/sprint0/SlidingAverageTest.java
@@ -49,6 +49,13 @@ void largeValuesDoNotOverflowTheWindowSum() {
         assertListDoubles(SlidingAverage.movingAverage(4, List.of(max, max, max, max), 2), max, max, max);
     }
 
+    @Test
+    void extremeValuesDoNotOverflowTheRollingUpdate() {
+        assertListDoubles(SlidingAverage.movingAverage(
+                2, List.of(Integer.MIN_VALUE, Integer.MAX_VALUE), 1),
+                Integer.MIN_VALUE, Integer.MAX_VALUE);
+    }
+
     private static void assertListDoubles(List<Double> actual, double... expected) {
         assertEquals(expected.length, actual.size(), "size");
         for (int i = 0; i < expected.length; i++) {
diff --git a/src/test/java/algorithms/sprint1/DistancesTest.java b/src/test/java/algorithms/sprint1/DistancesTest.java
new file mode 100644
index 0000000..b71466e
--- /dev/null
+++ b/src/test/java/algorithms/sprint1/DistancesTest.java
@@ -0,0 +1,38 @@
+package algorithms.sprint1;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.nio.charset.StandardCharsets;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+class DistancesTest {
+
+    @Test
+    void readsAndWritesThroughProvidedStreams() {
+        ByteArrayOutputStream output = new ByteArrayOutputStream();
+
+        Distances.run(input("5 0 1 4 9 0"), output);
+
+        assertEquals("0 1 2 1 0\n", output.toString(StandardCharsets.UTF_8));
+    }
+
+    @Test
+    void rejectsNegativeInputSize() {
+        assertThrows(IllegalArgumentException.class,
+                () -> Distances.run(input("-1"), new ByteArrayOutputStream()));
+    }
+
+    @Test
+    void rejectsExcessiveInputSize() {
+        assertThrows(IllegalArgumentException.class,
+                () -> Distances.run(input("100001"), new ByteArrayOutputStream()));
+    }
+
+    private static ByteArrayInputStream input(String value) {
+        return new ByteArrayInputStream(value.getBytes(StandardCharsets.UTF_8));
+    }
+}
diff --git a/src/test/java/algorithms/sprint5/SolutionTest.java b/src/test/java/algorithms/sprint5/SolutionTest.java
index ed6a4ee..82da844 100644
--- a/src/test/java/algorithms/sprint5/SolutionTest.java
+++ b/src/test/java/algorithms/sprint5/SolutionTest.java
@@ -51,6 +51,28 @@ void removeDeletesNodeWithTwoChildrenUsingPredecessor() {
         assertEquals(5, result.getRight().getValue());
     }
 
+    @Test
+    void removeHandlesDeepSkewedTreeWithoutOverflowingStack() {
+        Node root = node(null, null, 0);
+        Node current = root;
+        for (int value = 1; value < 100_000; value++) {
+            Node next = node(null, null, value);
+            current.setRight(next);
+            current = next;
+        }
+
+        Node result = Solution.remove(root, 99_999);
+
+        assertEquals(0, result.getValue());
+        assertNull(current.getRight());
+        current = result;
+        for (int value = 0; value < 99_999; value++) {
+            assertEquals(value, current.getValue());
+            current = current.getRight();
+        }
+        assertNull(current);
+    }
+
     private static Node node(Node left, Node right, int value) {
         return new Node(left, right, value);
     }
diff --git a/src/test/java/coderun/AverageElementTest.java b/src/test/java/coderun/AverageElementTest.java
index 0ae380b..4cf9df9 100644
--- a/src/test/java/coderun/AverageElementTest.java
+++ b/src/test/java/coderun/AverageElementTest.java
@@ -29,4 +29,31 @@ void shouldSolveFromReaderAndNormalizeWhitespace() throws IOException {
 
         assertEquals("5", output.toString());
     }
+
+    @Test
+    void shouldIgnoreInvalidOrInsufficientInput() throws IOException {
+        assertProducesNoOutput("");
+        assertProducesNoOutput("   ");
+        assertProducesNoOutput("5");
+        assertProducesNoOutput("1 invalid 3");
+    }
+
+    @Test
+    void shouldRejectAnOversizedInputLine() throws IOException {
+        assertProducesNoOutput("1 ".repeat(2049));
+    }
+
+    @Test
+    void shouldFindSecondValueWithoutSortingTheInput() {
+        assertEquals(2, AverageElement.average(new String[]{"4", "2", "3", "1"}));
+        assertEquals(1, AverageElement.average(new String[]{"1", "1"}));
+    }
+
+    private static void assertProducesNoOutput(String input) throws IOException {
+        StringWriter output = new StringWriter();
+
+        AverageElement.solve(new StringReader(input), output);
+
+        assertEquals("", output.toString());
+    }
 }
diff --git a/src/test/java/interview/MaximumSequenceWithOneZeroTest.java b/src/test/java/interview/MaximumSequenceWithOneZeroTest.java
index c30b939..1ce4edb 100644
--- a/src/test/java/interview/MaximumSequenceWithOneZeroTest.java
+++ b/src/test/java/interview/MaximumSequenceWithOneZeroTest.java
@@ -28,6 +28,11 @@ void shouldSplitSequenceOnTwoConsecutiveZeros() {
         assertEquals(3, maximumSequenceWithOneZero(List.of(1, 1, 0, 0, 1, 1, 1)));
     }
 
+    @Test
+    void shouldNotCountAcrossMultipleSeparatedZeros() {
+        assertEquals(2, maximumSequenceWithOneZero(List.of(1, 0, 1, 0, 1)));
+    }
+
     @Test
     void shouldCountAllOnesWhenThereIsNoZeroSeparator() {
         assertEquals(5, maximumSequenceWithOneZero(List.of(1, 1, 1, 1, 1)));
diff --git a/src/test/java/kyu3/RailFenceCipherTest.java b/src/test/java/kyu3/RailFenceCipherTest.java
index ccc1fcc..1ad9787 100644
--- a/src/test/java/kyu3/RailFenceCipherTest.java
+++ b/src/test/java/kyu3/RailFenceCipherTest.java
@@ -1,6 +1,7 @@
 package kyu3;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.stream.Stream;
 import org.junit.jupiter.api.Tag;
@@ -20,6 +21,12 @@ void shouldEncodeAndDecodeKnownExample() {
         assertEquals("WEAREDISCOVEREDFLEEATONCE", RailFenceCipher.decode(encoded, 3));
     }
 
+    @Test
+    void shouldDecodeWithoutAllocatingForUnusedRails() {
+        assertEquals("", RailFenceCipher.decode("", Integer.MAX_VALUE));
+        assertEquals("abc", RailFenceCipher.decode("abc", Integer.MAX_VALUE));
+    }
+
     @ParameterizedTest
     @MethodSource("roundTripCases")
     void shouldRoundTripPlainTextForDifferentRails(String text, int rails) {
@@ -28,6 +35,15 @@ void shouldRoundTripPlainTextForDifferentRails(String text, int rails) {
         assertEquals(text, RailFenceCipher.decode(encoded, rails));
     }
 
+    @ParameterizedTest
+    @MethodSource("invalidRailCounts")
+    void shouldRejectInvalidRailCounts(int rails) {
+        assertThrows(IllegalArgumentException.class,
+                () -> RailFenceCipher.encode("ABC", rails));
+        assertThrows(IllegalArgumentException.class,
+                () -> RailFenceCipher.decode("ABC", rails));
+    }
+
     private static Stream<Arguments> roundTripCases() {
         return Stream.of(
                 Arguments.of("", 2),
@@ -35,4 +51,12 @@ private static Stream<Arguments> roundTripCases() {
                 Arguments.of("Rail fence cipher keeps punctuation.", 5)
         );
     }
+
+    private static Stream<Arguments> invalidRailCounts() {
+        return Stream.of(
+                Arguments.of(-1),
+                Arguments.of(0),
+                Arguments.of(1)
+        );
+    }
 }
diff --git a/src/test/java/kyu5/PigLatinTest.java b/src/test/java/kyu5/PigLatinTest.java
index a6ee6ef..ba173ce 100644
--- a/src/test/java/kyu5/PigLatinTest.java
+++ b/src/test/java/kyu5/PigLatinTest.java
@@ -21,4 +21,11 @@ public class PigLatinTest {
     void smokeTestsShouldExecuteApi() {
         quality.SmokeMethodTestHarness.verify(kyu5.PigLatin.class);
     }
+
+    @Test
+    void handlesEmptyTokensWithoutThrowing() {
+        assertEquals("", pigIt(""));
+        assertEquals("eadinglay", pigIt(" leading"));
+        assertEquals("igPay atinlay", pigIt("Pig  latin"));
+    }
 }
diff --git a/src/test/java/kyu6/CamelCaseTest.java b/src/test/java/kyu6/CamelCaseTest.java
index e632d8d..1b80eb2 100644
--- a/src/test/java/kyu6/CamelCaseTest.java
+++ b/src/test/java/kyu6/CamelCaseTest.java
@@ -21,4 +21,14 @@ public class CamelCaseTest {
     void smokeTestsShouldExecuteApi() {
         quality.SmokeMethodTestHarness.verify(kyu6.CamelCase.class);
     }
+
+    @Test
+    void toCamelCaseShouldTreatRegexCharactersLiterally() {
+        assertEquals("[abc?def\\ghi$there", kyu6.CamelCase.toCamelCase("[abc ?def \\ghi $there"));
+    }
+
+    @Test
+    void toCamelCaseShouldHandleNullInput() {
+        assertEquals("", kyu6.CamelCase.toCamelCase(null));
+    }
 }
diff --git a/src/test/java/kyu6/EncryptThisTest.java b/src/test/java/kyu6/EncryptThisTest.java
index b11a2f5..1547c39 100644
--- a/src/test/java/kyu6/EncryptThisTest.java
+++ b/src/test/java/kyu6/EncryptThisTest.java
@@ -17,6 +17,17 @@
 import static kyu6.EncryptThis.*;
 @Tag("smoke")
 public class EncryptThisTest {
+    @Test
+    void encryptsWordsSeparatedByWhitespace() {
+        assertEquals("65 66", encryptThis("A  B"));
+        assertEquals("65 66", encryptThis(" A\tB "));
+    }
+
+    @Test
+    void returnsEmptyStringForBlankInput() {
+        assertEquals("", encryptThis(" \t\n"));
+    }
+
     @Test
     void smokeTestsShouldExecuteApi() {
         quality.SmokeMethodTestHarness.verify(kyu6.EncryptThis.class);
diff --git a/src/test/java/kyu6/NumberInExpandedFormTest.java b/src/test/java/kyu6/NumberInExpandedFormTest.java
index 08cc5a3..0ffe3ce 100644
--- a/src/test/java/kyu6/NumberInExpandedFormTest.java
+++ b/src/test/java/kyu6/NumberInExpandedFormTest.java
@@ -17,6 +17,11 @@
 import static kyu6.NumberInExpandedForm.*;
 @Tag("smoke")
 public class NumberInExpandedFormTest {
+    @Test
+    void zeroShouldRemainZero() {
+        assertEquals("0", expandedForm(0));
+    }
+
     @Test
     void smokeTestsShouldExecuteApi() {
         quality.SmokeMethodTestHarness.verify(kyu6.NumberInExpandedForm.class);
diff --git a/src/test/java/kyu6/TribonacciSequenceTest.java b/src/test/java/kyu6/TribonacciSequenceTest.java
index c308ca7..34e022b 100644
--- a/src/test/java/kyu6/TribonacciSequenceTest.java
+++ b/src/test/java/kyu6/TribonacciSequenceTest.java
@@ -31,5 +31,9 @@ void shouldRejectInvalidSignatureAndLength() {
         assertThrows(IllegalArgumentException.class, () -> TribonacciSequence.tribonacci(null, 1));
         assertThrows(IllegalArgumentException.class, () -> TribonacciSequence.tribonacci(new double[]{1, 1}, 2));
         assertThrows(IllegalArgumentException.class, () -> TribonacciSequence.tribonacci(new double[]{1, 1, 1}, -1));
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> TribonacciSequence.tribonacci(new double[]{1, 1, 1}, Integer.MAX_VALUE)
+        );
     }
 }
diff --git a/src/test/java/kyu7/MaskifyTest.java b/src/test/java/kyu7/MaskifyTest.java
index 803a117..aa874aa 100644
--- a/src/test/java/kyu7/MaskifyTest.java
+++ b/src/test/java/kyu7/MaskifyTest.java
@@ -17,6 +17,11 @@
 import static kyu7.Maskify.*;
 @Tag("smoke")
 public class MaskifyTest {
+    @Test
+    void nullInputShouldRemainNull() {
+        assertNull(maskify(null));
+    }
+
     @Test
     void smokeTestsShouldExecuteApi() {
         quality.SmokeMethodTestHarness.verify(kyu7.Maskify.class);
diff --git a/src/test/java/other/RecursionMaxTest.java b/src/test/java/other/RecursionMaxTest.java
index dd6f531..75cb27c 100644
--- a/src/test/java/other/RecursionMaxTest.java
+++ b/src/test/java/other/RecursionMaxTest.java
@@ -3,6 +3,7 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.stream.Stream;
 import org.junit.jupiter.api.Tag;
@@ -29,6 +30,13 @@ void shouldNotMutateInputList() {
         assertEquals(List.of(3, 8, 1, 5), values);
     }
 
+    @Test
+    void shouldHandleLargeListsWithoutExhaustingTheStack() {
+        List<Integer> values = Collections.nCopies(100_000, 42);
+
+        assertEquals(42, RecursionMax.recursionMax(values));
+    }
+
     private static Stream<Arguments> maxCases() {
         return Stream.of(
                 Arguments.of(null, -1),
diff --git a/src/test/java/quality/SmokeMethodTestHarness.java b/src/test/java/quality/SmokeMethodTestHarness.java
index 0b931ef..61d420d 100644
--- a/src/test/java/quality/SmokeMethodTestHarness.java
+++ b/src/test/java/quality/SmokeMethodTestHarness.java
@@ -58,7 +58,7 @@ public static void verify(Class<?> clazz) {
         }
 
         String summary = String.format("%s attempted=%d failed=%d", clazz.getSimpleName(), attempted, failed);
-        Assertions.assertTrue(failed < attempted, () -> "Every executable smoke path failed: " + summary);
+        Assertions.assertEquals(0, failed, () -> "Executable smoke path failed: " + summary);
     }
 
     private static Object resolveTargetInstance(Method method, Class<?> clazz) throws Exception {
diff --git a/src/test/java/quality/SmokeMethodTestHarnessTest.java b/src/test/java/quality/SmokeMethodTestHarnessTest.java
index 8fef2ae..8187185 100644
--- a/src/test/java/quality/SmokeMethodTestHarnessTest.java
+++ b/src/test/java/quality/SmokeMethodTestHarnessTest.java
@@ -1,6 +1,5 @@
 package quality;
 
-import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
@@ -13,8 +12,8 @@ void rejectsClassesWhoseEveryExecutablePathFails() {
     }
 
     @Test
-    void acceptsAClassWhenAtLeastOneExecutablePathSucceeds() {
-        assertDoesNotThrow(() -> SmokeMethodTestHarness.verify(PartiallyWorks.class));
+    void rejectsAClassWhenAnyExecutablePathFails() {
+        assertThrows(AssertionError.class, () -> SmokeMethodTestHarness.verify(PartiallyWorks.class));
     }
 
     private static final class AlwaysFails {