MergeSortInPlace.java

import java.util.Arrays;

public class MergeSortInPlace {

    public static void main(String[] args) {

        int[] arr = { 4, 6, 7, 4, 3, 1, 1 };
        MergeSortInPla(arr, 0, arr.length);
        System.out.println(Arrays.toString(arr));
    }

    private static void MergeSortInPla(int[] arr, int s, int e) {
        if (e - s == 1)
            return;

        int m = s + (e - s) / 2;
        MergeSortInPla(arr, s, m);
        MergeSortInPla(arr, m, e);

        Merge(arr, s, m, e);
    }

    private static void Merge(int[] arr, int s, int m, int e) {

        int[] ans = new int[e - s];
        int i = s;
        int j = m;
        int index = 0;

        while (i < m && j < e) {
            if (arr[i] < arr[j]) {
                ans[index] = arr[i];
                i++;
                index++;
            } else {
                ans[index] = arr[j];
                j++;
                index++;
            }
        }

        while (i < m) {
            ans[index] = arr[i];
            i++;
            index++;
        }

        while (j < e) {
            ans[index] = arr[j];
            j++;
            index++;
        }

        for (int a = 0; a < ans.length; a++) {
            arr[s + a] = ans[a];
        }
    }
}