归并排序模板。
#include <bits/stdc++.h>
using namespace std;
long long cnt = 0;
int a[1000007], b[1000007];
void merge_sort(int l, int r) {
if (l == r) return;
int mid = l + r >> 1;
merge_sort(l, mid);
merge_sort(mid + 1, r);
int i = l, j = mid + 1, k = l;
while (i <= mid && j <= r) {
if (a[i] <= a[j]) b[k++] = a[i++];
else b[k++] = a[j++], cnt += mid - i + 1;///逆序数,表示从i到mid的数字大于a[j]都要算一遍
}
while (i <= mid) b[k++] = a[i++];
while (j <= r) b[k++] = a[j++];
for (int i = l;i <= r;i++) a[i] = b[i];
}
int main() {
std::ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
int n;
cin >> n;
for (int i = 0;i < n;i++) cin >> a[i];
merge_sort(0, n - 1);
cout << cnt << '\n';
for (int i = 0;i < n;i++) cout << a[i] << ' ';
cout << '\n';
return 0;
}