歸併排序

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

    public static void mergeSort(int[] arr, int l, int r) {
        if (l == r) {
            return;
        }
        int
 
 mid = l + ((r - l) >> 1);
        mergeSort(arr, l, mid);
        mergeSort(arr, mid + 1, r);
        merge(arr, l, mid, r);
    }

    public static void merge(int[] arr, int l, int m, int r) {
        int [] help = new int [r - l + 1];
        int i = 0;
        int p1 = l;
        int p2 = m + 1
 
;
        while(p1 <= m && p2 <= r){
            help[i++] = arr[p1] < arr[p2] ? arr[p1++] : arr[p2++];
        }
        while(p1 <= m){
            help[i++] = arr[p1++];
        }
        while(p2 <= r){
            help[i++] = arr[p2++];
        }
        for(i = 0; i < help.length; i++){
            arr[l+i] = help[i];
        }
    }