归并排序(递归实现)

public static void mergeSort1(int[] arr){
    if(arr ==null || arr.length <2){
        return;
    }

    process(arr, 0 , arr.length-1);
}

public static void process(int[] arr ,int L ,int R){
    if(L ==R){
        return;
    }
    int mid = L +((R-L)>>1);
    process(arr , L , mid);
    process(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];
    }
}