题目训练网址（密码hpuacm）：https://vjudge.net/contest/243680

 

先说一下归并排序求逆序数法

什么事逆序数   戳这里

C题模板（归并排序求逆序数）

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

vector<int> v;
LL merge_sort( vector<int> &v )
{
    int n = v.size();
    if( n <= 1 )
        return 0;
    LL cnt = 0;
    vector<int> vb(v.begin(), v.begin() + n/2);
    vector<int> ve(v.begin() + n/2, v.end());
    cnt += merge_sort(vb);
    cnt += merge_sort(ve);
    int ai, bi, ci;
    ai = bi = ci = 0;
    while( ai < n )
    {
        if( bi < vb.size() && (ci == ve.size() || vb[bi] <= ve[ci]) )
            v[ai++] = vb[bi++];
        else
        {
            v[ai++] = ve[ci++];
            cnt += n/2 - bi;
        }
    }
    return cnt;
}

int main()
{
    int n;
    while( cin >> n )
    {
        int num;
        v.clear();
        for( int i=0; i<n; i++ )
        {
            scanf("%d", &num );
            v.push_back(num);
        }
        printf("%d\n", merge_sort(v) );

    }

    return 0;
}