数列中逆序对的数量

给定一个长度为 n的整数数列，计算数列中的逆序对的数量。

#include <iostream>
using namespace std;
const int N = 100010;
int n, a[N];

long long sort (int l, int r) {
    if (l >= r) return 0;
    int mid = l + r >> 1;
    long long ans = sort(l, mid) + sort(mid + 1, r);
    
    int t[r - l + 1];
    int i = l, j = mid + 1, k = 0;
    while (i <= mid && j <= r) {
        if (a[i] <= a[j]) t[k++] = a[i++];
        else {
            ans += mid - i + 1;
            t[k++] = a[j++];
        }
    }
    
    while (i <= mid) t[k++] = a[i++];
    while (j <= r) t[k++] = a[j++];
    
    k = 0;
    for (int i = l; i <= r; i++, k++) a[i] = t[k];
    return ans;
}

int main() {
    scanf ("%d", &n);
    for (int i = 0; i < n; i++) scanf ("%d", &a[i]);
     
    long long ans = sort (0, n - 1);
    printf ("%lld", ans);
    return 0;
}