代码
//
// Created by Genes on 2020/11/16.
//
// 归并
// 逆序对的数量
#include<iostream>
using namespace std;
const int N = 1e5 + 10;
typedef long long ll;
int n;
int q[N], tmp[N];
ll ans;
void merge_sort(int l, int r) {
if (l >= r) {
return;
}
int res = 0;
int mid = (l + r) >> 1;
merge_sort(l, mid), merge_sort(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++];
ans += mid - i + 1;
}
}
while (i <= mid) {
tmp[k++] = q[i++];
}
while (j <= r) {
tmp[k++] = q[j++];
}
for (int i = l, j = 0; i <= r; i++, j++) {
q[i] = tmp[j];
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
cin >> n;
for (int i = 0; i < n; i++) {
cin >> q[i];
}
merge_sort(0, n - 1);
cout << ans << endl;
return 0;
}
棒