求助求助，一半AC，一半WA

#include <iostream>
#include <vector>
using namespace std;

void Merge(vector<int> &r, int s, int m, int t, int &sum) {
    vector<int> r1(t - s + 1);
    int i = s, j = m + 1, k = 0;
    while (i <= m && j <= t) {
        if (r[i] <= r[j]) 
            r1[k++] = r[i++];
        else {
            r1[k++] = r[j++];
            sum += (m - i + 1);
        }
    } 
    while (i <= m) 
        r1[k++] = r[i++];
    while (j <= t) 
        r1[k++] = r[j++];
    for (int j = 0, i = s; j < k; i++, j++) {
        r[i] = r1[j];
    }
}

void MergeSort(vector<int> &r, int s, int t, int &sum) {
    if (s >= t) return;
    int m = (s + t) / 2;
    MergeSort(r, s, m, sum);
    MergeSort(r, m + 1, t, sum);
    Merge(r, s, m, t, sum);
}

int main() {
    int n;
    cin >> n;
    vector<int> a(n); 
    int sum = 0;
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    MergeSort(a, 0, n - 1, sum);
    cout << sum<<endl;
    return 0;
}