写一个二路归并算法用java

public class MergeSort { public static void mergeSort(int[] arr) { if (arr == null || arr.length < 2) { return; } mergeSort(arr, 0, arr.length - 1); }

private static void mergeSort(int[] arr, int left, int right) {
    if (left == right) {
        return;
    }
    int mid = left + ((right - left) >> 1);
    mergeSort(arr, left, mid);
    mergeSort(arr, mid + 1, right);
    merge(arr, left, mid, right);
}

private static void merge(int[] arr, int left, int mid, int right) {
    int[] help = new int[right - left + 1];
    int i = 0;
    int p1 = left;
    int p2 = mid + 1;
    while (p1 <= mid && p2 <= right) {
        help[i++] = arr[p1] < arr[p2] ? arr[p1++] : arr[p2++];
    }
    while (p1 <= mid) {
        help[i++] = arr[p1++];
    }
    while (p2 <= right) {
        help[i++] = arr[p2++];
    }
    for (i = 0; i < help.length; i++) {
        arr[left + i] = help[i];
    }
}

}