归并排序求逆序对的数量

归并排序求逆序对的数量

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

const int N = 1e5 + 10;

int n;
int a[N], tmp[N];

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

    return res;
}
signed main(){
    scanf("%d", &n);
    for(int i = 0; i < n; ++i) scanf("%d", &a[i]);

    printf("%lld\n", merge_sort(0, n - 1));
    
    return 0;
}