[模板] 归并排序

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;
}