问题

最后return res 忘了太蠢了 本来写了 结果前面 while（l >= r) return 报错就全部删了

算法1

C++ 代码

#include <iostream>
using namespace std;

typedef long long LL;
int n;
const int N = 100010;
int q[N],tmp[N];

LL merge_sort(int q[], int l, int r)
{
    if (l >= r) return 0;

    int mid = l + r >> 1;
    LL res = merge_sort(q, l, mid) + merge_sort(q, mid + 1, r);

    int k = 0, i = l, j = mid + 1;
    while (i <= mid && j <= r)
        if (q[i] <= q[j]) tmp[k ++ ] = q[i ++ ];
        else {
            tmp[k ++ ] = q[j ++ ];
            res += (mid - i + 1) ;
        }

    while (i <= mid) tmp[k ++ ] = q[i ++ ];
    while (j <= r) tmp[k ++ ] = q[j ++ ];

    for (i = l, j = 0; i <= r; i ++, j ++ ) q[i] = tmp[j];

    return res;
}

int main(){
    cin >> n;

    for(int i  =0 ; i < n ;i++)  cin >> q[i];

    cout << merge_sort(q, 0 , n -1) << endl;

    //printf("%d",res);

    return 0;

}