Implement Merge Sort algorithm in Java

Sorting_Algorithms_JAVA/merge_sort.java

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+import java.util.Arrays;
+
+public class MergeSort {
+
+    /**
+     * The main public method to sort an array.
+     * It calls the recursive sort function.
+     */
+    public void sort(int[] arr) {
+        if (arr == null || arr.length < 2) {
+            return; // Already sorted
+        }
+        sortRecursive(arr, 0, arr.length - 1);
+    }
+
+    /**
+     * The recursive "divide" part of the algorithm.
+     * @param arr   The array to be sorted.
+     * @param left  The starting index.
+     * @param right The ending index.
+     */
+    private void sortRecursive(int[] arr, int left, int right) {
+        // Base case: if the array has 1 or 0 elements, it's sorted.
+        if (left < right) {
+            // 1. Find the middle point to divide the array
+            int middle = (left + right) / 2;
+
+            // 2. Conquer: Recursively sort the left half
+            sortRecursive(arr, left, middle);
+
+            // 3. Conquer: Recursively sort the right half
+            sortRecursive(arr, middle + 1, right);
+
+            // 4. Combine: Merge the two sorted halves
+            merge(arr, left, middle, right);
+        }
+    }
+
+    /**
+     * The "combine" part of the algorithm.
+     * Merges two sorted subarrays arr[left...middle] and arr[middle+1...right].
+     */
+    private void merge(int[] arr, int left, int middle, int right) {
+        // Find sizes of the two temporary subarrays
+        int n1 = middle - left + 1;
+        int n2 = right - middle;
+
+        // Create temporary arrays
+        int[] L = new int[n1];
+        int[] R = new int[n2];
+
+        // Copy data into the temporary arrays
+        for (int i = 0; i < n1; ++i) {
+            L[i] = arr[left + i];
+        }
+        for (int j = 0; j < n2; ++j) {
+            R[j] = arr[middle + 1 + j];
+        }
+
+        // --- Merge the temporary arrays back into arr[left...right] ---
+
+        int i = 0; // Initial index of first subarray (L)
+        int j = 0; // Initial index of second subarray (R)
+        int k = left; // Initial index of merged subarray (arr)
+
+        while (i < n1 && j < n2) {
+            if (L[i] <= R[j]) {
+                arr[k] = L[i];
+                i++;
+            } else {
+                arr[k] = R[j];
+                j++;
+            }
+            k++;
+        }
+
+        // Copy any remaining elements of L[]
+        while (i < n1) {
+            arr[k] = L[i];
+            i++;
+            k++;
+        }
+
+        // Copy any remaining elements of R[]
+        while (j < n2) {
+            arr[k] = R[j];
+            j++;
+            k++;
+        }
+    }
+
+    /**
+     * A main method to test the sort.
+     */
+    public static void main(String[] args) {
+        int[] data = {12, 11, 13, 5, 6, 7, 22, -3};
+
+        System.out.println("Original array: " + Arrays.toString(data));
+
+        MergeSort sorter = new MergeSort();
+        sorter.sort(data);
+
+        System.out.println("Sorted array:   " + Arrays.toString(data));
+    }
+}