python/TangDou/AcWing_TiGao/T6/PaiXu/107.cpp

#include <bits/stdc++.h>
using namespace std;

typedef long long LL;
const int N = 500010;
int n;
LL q[N], t[N];
LL ans; // 结果

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

    int i = l, j = mid + 1, k = 0;
    while (i <= mid && j <= r)
        if (q[i] <= q[j])
            t[k++] = q[i++];
        else {
            ans += mid - i + 1; // 如果q[i]>q[j],那么 i~mid之间所有数字，都>q[j]
            t[k++] = q[j++];
        }
    // 清理剩余
    while (i <= mid) t[k++] = q[i++];
    while (j <= r) t[k++] = q[j++];
    // 抄回来
    for (i = l, j = 0; i <= r; i++, j++) q[i] = t[j];
}

int main() {
    while (cin >> n, n) {
        for (int i = 1; i <= n; i++) cin >> q[i];
        ans = 0;
        merge_sort(q, 1, n);
        printf("%lld\n", ans);
    }
    return 0;
}