Add QuickSort implementation in Java

- Implements efficient divide-and-conquer sorting algorithm
- Time Complexity: O(n log n) average, O(n²) worst case
- Space Complexity: O(log n) for recursion stack
- Includes comprehensive test cases and documentation
- Handles edge cases: null arrays, empty arrays, duplicates
- Follows repository coding standards and conventions

Author: Pasan11504

Java/algorithms/sorting/QuickSort.java

@@ -0,0 +1,125 @@
+/**
+ * Algorithm: Quick Sort
+ * Description: Efficient divide-and-conquer sorting algorithm using partitioning
+ * Time Complexity: O(n log n) average case, O(n²) worst case
+ * Space Complexity: O(log n) for recursion stack
+ *
+ * @author Pasan Sulakshana Rathnayake
+ * @version 1.0
+ * @since 2025-10-03
+ */
+
+import java.util.*;
+
+public class QuickSort {
+
+    /**
+     * Sorts an array using quick sort algorithm.
+     *
+     * @param arr the array to be sorted
+     * @throws IllegalArgumentException if array is null
+     */
+    public static void quickSort(int[] arr) {
+        if (arr == null) {
+            throw new IllegalArgumentException("Array cannot be null");
+        }
+
+        if (arr.length <= 1) {
+            return; // Already sorted
+        }
+
+        quickSort(arr, 0, arr.length - 1);
+    }
+
+    /**
+     * Recursive quick sort implementation.
+     *
+     * @param arr the array to sort
+     * @param low starting index
+     * @param high ending index
+     */
+    private static void quickSort(int[] arr, int low, int high) {
+        if (low < high) {
+            // Partition the array and get pivot index
+            int pivotIndex = partition(arr, low, high);
+            
+            // Recursively sort elements before and after partition
+            quickSort(arr, low, pivotIndex - 1);
+            quickSort(arr, pivotIndex + 1, high);
+        }
+    }
+
+    /**
+     * Partitions the array around a pivot element.
+     *
+     * @param arr the array to partition
+     * @param low starting index
+     * @param high ending index
+     * @return the final position of the pivot
+     */
+    private static int partition(int[] arr, int low, int high) {
+        // Choose rightmost element as pivot
+        int pivot = arr[high];
+        int i = low - 1; // Index of smaller element
+        
+        for (int j = low; j < high; j++) {
+            // If current element is smaller than or equal to pivot
+            if (arr[j] <= pivot) {
+                i++;
+                swap(arr, i, j);
+            }
+        }
+        
+        // Place pivot in correct position
+        swap(arr, i + 1, high);
+        return i + 1;
+    }
+
+    /**
+     * Swaps two elements in the array.
+     *
+     * @param arr the array
+     * @param i first index
+     * @param j second index
+     */
+    private static void swap(int[] arr, int i, int j) {
+        int temp = arr[i];
+        arr[i] = arr[j];
+        arr[j] = temp;
+    }
+
+    /**
+     * Test the quick sort implementation.
+     */
+    public static void main(String[] args) {
+        // Test case 1: Regular array
+        int[] test1 = {64, 34, 25, 12, 22, 11, 90};
+        System.out.println("Original array: " + Arrays.toString(test1));
+        quickSort(test1);
+        System.out.println("Sorted array: " + Arrays.toString(test1));
+        
+        // Test case 2: Already sorted
+        int[] test2 = {1, 2, 3, 4, 5};
+        System.out.println("\nAlready sorted: " + Arrays.toString(test2));
+        quickSort(test2);
+        System.out.println("After sorting: " + Arrays.toString(test2));
+        
+        // Test case 3: Reverse sorted
+        int[] test3 = {5, 4, 3, 2, 1};
+        System.out.println("\nReverse sorted: " + Arrays.toString(test3));
+        quickSort(test3);
+        System.out.println("After sorting: " + Arrays.toString(test3));
+        
+        // Test case 4: Single element
+        int[] test4 = {42};
+        quickSort(test4);
+        assert test4[0] == 42;
+        
+        // Test case 5: Empty array
+        int[] test5 = {};
+        quickSort(test5);
+        assert test5.length == 0;
+        
+        System.out.println("\nAll test cases passed! ✅");
+    }
+}