void merge_sort(int A[], int l, int r)
{
    if(l>=r) return;
    int mid = l+r>>1;

    merge_sort(A,l,mid), merge_sort(A,mid+1,r);

    int i = l, j = mid+1, k=0; //有3个索引
                                  //有两段数组，一个是l ~ mid，另一个是mid+1 ~ r；
                                  //k记录新数组
    while(i<=mid && j<=r) //记得先while循环，再if比较
    {
        if(A[i]<A[j]) tmp[k++] = A[i++];
        else tmp[k++] = A[j++];
    }
    while(i<=mid) tmp[k++] = A[i++];
    while(j<=r) tmp[k++] = A[j++];

    for(int i=l, k=0; i<=r; i ++)  //有2个索引
    {
        A[i] = tmp[k++];
    }
}