MergeSort.java

public class MergeSort {

    static void TopDownMergeSort(int[] a, int[] b, int n) {
        CopyArray(a, 0, n, b); 
        TopDownSplitMerge(b, 0, n, a);
    }

    static void TopDownSplitMerge(int[] b, int left, int right, int[] a) {
        if (right - left < 1)
            return;
        int mid;
        mid = (int) Math.floor((right + left) / 2);
        TopDownSplitMerge(a, left, mid, b);
        TopDownSplitMerge(a, mid + 1, right, b);
        TopDownMerge(b, left, mid, right, a);
    }

    static void TopDownMerge(int[] a, int left, int mid, int right, int[] b) {
        int i = left;
        int j = mid + 1;

        for (int k = left; k <= right; k++) {
            if (i <= mid && (j > right || a[i] <= a[j])) {
                b[k] = a[i];
                i++;
            } else {
                b[k] = a[j];
                j++;
            }
        }
    }

    static void CopyArray(int[] a, int left, int right, int[] b) {
        for (int k = left; k <= right; k++) {
            b[k] = a[k];
        }
    }

    public static void main(String[] args) {
        int[] a = { 4, 2, 11, 3, 6, 9, 1 };
        int[] b = new int[a.length];
        TopDownMergeSort(a, b, a.length - 1);
    }

}