（分治）逆序数计数

#include<bits/stdc++.h>
#include <iostream>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
vector <int> A;
int n,tem;
ll merge_count(vector <int> &a)
{
    int m=a.size();
    if(m<=1)
        return 0;

    ll cnt=0;
    vector <int> b(a.begin(),a.begin()+m/2);
    vector <int> c(a.begin()+m/2,a.end());
    cnt+=merge_count(b);
    cnt+=merge_count(c);
    int ai=0,bi=0,ci=0;
    while(ai<m)
    {
        if(bi<b.size()&&(ci==c.size()||b[bi]<=c[ci]))
        {
            a[ai++]=b[bi++];
        }
        else
        {
            cnt+=m/2-bi;
            a[ai++]=c[ci++];
        }
    }
    return cnt;

}
int main()
{
    scanf("%d",&n);
    for(int i=0;i<n;i++)
        {
            scanf("%d",&tem);
            A.push_back(tem);
        }
    printf("%lld\n",merge_count(A));
    return 0;
}