Merge branch 'master' into master

Author: DenizAltunkapan

.github/workflows/infer.yml

@@ -33,7 +33,7 @@ jobs:
 
       - name: Cache infer build
         id: cache-infer
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: infer
           key: ${{ runner.os }}-infer-${{ env.year_week }}

DIRECTORY.md

@@ -822,6 +822,7 @@
           - 📄 [Upper](src/main/java/com/thealgorithms/strings/Upper.java)
           - 📄 [ValidParentheses](src/main/java/com/thealgorithms/strings/ValidParentheses.java)
           - 📄 [WordLadder](src/main/java/com/thealgorithms/strings/WordLadder.java)
+          - 📄 [ZAlgorithm](src/main/java/com/thealgorithms/strings/ZAlgorithm.java)
           - 📁 **zigZagPattern**
             - 📄 [ZigZagPattern](src/main/java/com/thealgorithms/strings/zigZagPattern/ZigZagPattern.java)
         - 📁 **tree**
@@ -1579,6 +1580,7 @@
           - 📄 [UpperTest](src/test/java/com/thealgorithms/strings/UpperTest.java)
           - 📄 [ValidParenthesesTest](src/test/java/com/thealgorithms/strings/ValidParenthesesTest.java)
           - 📄 [WordLadderTest](src/test/java/com/thealgorithms/strings/WordLadderTest.java)
+          - 📄 [ZAlgorithmTest](src/test/java/com/thealgorithms/strings/ZAlgorithmTest.java)
           - 📁 **zigZagPattern**
             - 📄 [ZigZagPatternTest](src/test/java/com/thealgorithms/strings/zigZagPattern/ZigZagPatternTest.java)
         - 📁 **tree**

pom.xml

@@ -112,7 +112,7 @@
                     <dependency>
                     <groupId>com.puppycrawl.tools</groupId>
                     <artifactId>checkstyle</artifactId>
-                    <version>12.2.0</version>
+                    <version>12.3.0</version>
                     </dependency>
                 </dependencies>
             </plugin>

src/main/java/com/thealgorithms/datastructures/trees/BinaryTreeToString.java

@@ -0,0 +1,100 @@
+package com.thealgorithms.datastructures.trees;
+
+/**
+ * Leetcode 606: Construct String from Binary Tree:
+ * https://leetcode.com/problems/construct-string-from-binary-tree/
+ *
+ * Utility class to convert a {@link BinaryTree} into its string representation.
+ * <p>
+ * The conversion follows a preorder traversal pattern (root → left → right)
+ * and uses parentheses to denote the tree structure.
+ * Empty parentheses "()" are used to explicitly represent missing left children
+ * when a right child exists, ensuring the structure is unambiguous.
+ * </p>
+ *
+ * <h2>Rules:</h2>
+ * <ul>
+ * <li>Each node is represented as {@code (value)}.</li>
+ * <li>If a node has only a right child, include {@code ()} before the right
+ * child
+ * to indicate the missing left child.</li>
+ * <li>If a node has no children, it appears as just {@code (value)}.</li>
+ * <li>The outermost parentheses are removed from the final string.</li>
+ * </ul>
+ *
+ * <h3>Example:</h3>
+ *
+ * <pre>
+ *     Input tree:
+ *           1
+ *          / \
+ *         2   3
+ *          \
+ *           4
+ *
+ *     Output string:
+ *     "1(2()(4))(3)"
+ * </pre>
+ *
+ * <p>
+ * This implementation matches the logic from LeetCode problem 606:
+ * <i>Construct String from Binary Tree</i>.
+ * </p>
+ *
+ * @author Muhammad Junaid
+ * @see BinaryTree
+ */
+public class BinaryTreeToString {
+
+    /** String builder used to accumulate the string representation. */
+    private StringBuilder sb;
+
+    /**
+     * Converts a binary tree (given its root node) to its string representation.
+     *
+     * @param root the root node of the binary tree
+     * @return the string representation of the binary tree, or an empty string if
+     *         the tree is null
+     */
+    public String tree2str(BinaryTree.Node root) {
+        if (root == null) {
+            return "";
+        }
+
+        sb = new StringBuilder();
+        dfs(root);
+
+        // Remove the leading and trailing parentheses added by the root call
+        return sb.substring(1, sb.length() - 1);
+    }
+
+    /**
+     * Performs a recursive depth-first traversal to build the string.
+     * Each recursive call appends the node value and its children (if any)
+     * enclosed in parentheses.
+     *
+     * @param node the current node being processed
+     */
+    private void dfs(BinaryTree.Node node) {
+        if (node == null) {
+            return;
+        }
+
+        sb.append("(").append(node.data);
+
+        // Recursively build left and right subtrees
+        if (node.left != null) {
+            dfs(node.left);
+        }
+
+        // Handle the special case: right child exists but left child is null
+        if (node.right != null && node.left == null) {
+            sb.append("()");
+            dfs(node.right);
+        } else if (node.right != null) {
+            dfs(node.right);
+        }
+
+        sb.append(")");
+    }
+}

src/main/java/com/thealgorithms/maths/ExtendedEuclideanAlgorithm.java

@@ -0,0 +1,48 @@
+package com.thealgorithms.maths;
+
+/**
+ * In mathematics, the extended Euclidean algorithm is an extension to the
+ * Euclidean algorithm, and computes, in addition to the greatest common divisor
+ * (gcd) of integers a and b, also the coefficients of Bézout's identity, which
+ * are integers x and y such that ax + by = gcd(a, b).
+ *
+ * <p>
+ * For more details, see
+ * https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm
+ */
+public final class ExtendedEuclideanAlgorithm {
+
+    private ExtendedEuclideanAlgorithm() {
+    }
+
+    /**
+     * This method implements the extended Euclidean algorithm.
+     *
+     * @param a The first number.
+     * @param b The second number.
+     * @return An array of three integers:
+     *         <ul>
+     *         <li>Index 0: The greatest common divisor (gcd) of a and b.</li>
+     *         <li>Index 1: The value of x in the equation ax + by = gcd(a, b).</li>
+     *         <li>Index 2: The value of y in the equation ax + by = gcd(a, b).</li>
+     *         </ul>
+     */
+    public static long[] extendedGCD(long a, long b) {
+        if (b == 0) {
+            // Base case: gcd(a, 0) = a. The equation is a*1 + 0*0 = a.
+            return new long[] {a, 1, 0};
+        }
+
+        // Recursive call
+        long[] result = extendedGCD(b, a % b);
+        long gcd = result[0];
+        long x1 = result[1];
+        long y1 = result[2];
+
+        // Update coefficients using the results from the recursive call
+        long x = y1;
+        long y = x1 - a / b * y1;
+
+        return new long[] {gcd, x, y};
+    }
+}

src/main/java/com/thealgorithms/maths/LuckyNumber.java

@@ -0,0 +1,78 @@
+package com.thealgorithms.maths;
+
+/**
+ * In number theory, a lucky number is a natural number in a set which is generated by a certain "sieve".
+ * This sieve is similar to the sieve of Eratosthenes that generates the primes,
+ * but it eliminates numbers based on their position in the remaining set,
+ * instead of their value (or position in the initial set of natural numbers).
+ *
+ * Wiki: https://en.wikipedia.org/wiki/Lucky_number
+ */
+public final class LuckyNumber {
+
+    private LuckyNumber() {
+    }
+
+    // Common validation method
+    private static void validatePositiveNumber(int number) {
+        if (number <= 0) {
+            throw new IllegalArgumentException("Number must be positive.");
+        }
+    }
+
+    // Function to check recursively for Lucky Number
+    private static boolean isLuckyRecursiveApproach(int n, int counter) {
+        // Base case: If counter exceeds n, number is lucky
+        if (counter > n) {
+            return true;
+        }
+
+        // If number is eliminated in this step, it's not lucky
+        if (n % counter == 0) {
+            return false;
+        }
+
+        // Calculate new position after removing every counter-th number
+        int newNumber = n - (n / counter);
+
+        // Recursive call for next round
+        return isLuckyRecursiveApproach(newNumber, counter + 1);
+    }
+
+    /**
+     * Check if {@code number} is a Lucky number or not using recursive approach
+     *
+     * @param number the number
+     * @return {@code true} if {@code number} is a Lucky number, otherwise false
+     */
+    public static boolean isLuckyNumber(int number) {
+        validatePositiveNumber(number);
+        int counterStarting = 2;
+        return isLuckyRecursiveApproach(number, counterStarting);
+    }
+
+    /**
+     * Check if {@code number} is a Lucky number or not using iterative approach
+     *
+     * @param number the number
+     * @return {@code true} if {@code number} is a Lucky number, otherwise false
+     */
+    public static boolean isLucky(int number) {
+        validatePositiveNumber(number);
+
+        int counter = 2; // Position starts from 2 (since first elimination happens at 2)
+        int position = number; // The position of the number in the sequence
+
+        while (counter <= position) {
+            if (position % counter == 0) {
+                return false;
+            } // Number is eliminated
+
+            // Update the position of n after removing every counter-th number
+            position = position - (position / counter);
+            counter++;
+        }
+
+        return true; // Survives all eliminations → Lucky Number
+    }
+}

src/main/java/com/thealgorithms/physics/ThinLens.java

@@ -0,0 +1,74 @@
+package com.thealgorithms.physics;
+
+/**
+ * Implements the Thin Lens Formula used in ray optics:
+ *
+ * <pre>
+ *     1/f = 1/v + 1/u
+ * </pre>
+ *
+ * where:
+ * <ul>
+ *   <li>f = focal length</li>
+ *   <li>u = object distance</li>
+ *   <li>v = image distance</li>
+ * </ul>
+ *
+ * Uses the Cartesian sign convention.
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Thin_lens">Thin Lens</a>
+ */
+public final class ThinLens {
+
+    private ThinLens() {
+        throw new AssertionError("No instances.");
+    }
+
+    /**
+     * Computes the image distance using the thin lens formula.
+     *
+     * @param focalLength focal length of the lens (f)
+     * @param objectDistance object distance (u)
+     * @return image distance (v)
+     * @throws IllegalArgumentException if focal length or object distance is zero
+     */
+    public static double imageDistance(double focalLength, double objectDistance) {
+
+        if (focalLength == 0 || objectDistance == 0) {
+            throw new IllegalArgumentException("Focal length and object distance must be non-zero.");
+        }
+
+        return 1.0 / ((1.0 / focalLength) - (1.0 / objectDistance));
+    }
+
+    /**
+     * Computes magnification of the image.
+     *
+     * <pre>
+     *     m = v / u
+     * </pre>
+     *
+     * @param imageDistance image distance (v)
+     * @param objectDistance object distance (u)
+     * @return magnification
+     * @throws IllegalArgumentException if object distance is zero
+     */
+    public static double magnification(double imageDistance, double objectDistance) {
+
+        if (objectDistance == 0) {
+            throw new IllegalArgumentException("Object distance must be non-zero.");
+        }
+
+        return imageDistance / objectDistance;
+    }
+
+    /**
+     * Determines whether the image formed is real or virtual.
+     *
+     * @param imageDistance image distance (v)
+     * @return {@code true} if image is real, {@code false} if virtual
+     */
+    public static boolean isRealImage(double imageDistance) {
+        return imageDistance > 0;
+    }
+}

src/main/java/com/thealgorithms/recursion/FibonacciSeries.java

@@ -12,10 +12,12 @@ private FibonacciSeries() {
         throw new UnsupportedOperationException("Utility class");
     }
     public static int fibonacci(int n) {
+        if (n < 0) {
+            throw new IllegalArgumentException("n must be a non-negative integer");
+        }
         if (n <= 1) {
             return n;
-        } else {
-            return fibonacci(n - 1) + fibonacci(n - 2);
         }
+        return fibonacci(n - 1) + fibonacci(n - 2);
     }
 }