codeforce 660D Number of Parallelograms

题意：询问多少个矩形。

统计横纵坐标差，放进vector中

#include<cstdio>
#include<cstring>
#include<iostream>
#include<queue>
#include<stack>
#include<bits/stdc++.h>
using namespace std;
#define LL long long
int x[2010],y[2010];
vector<pair<int,int> >v;
int main(){
    int n;
    while(~scanf("%d",&n)){
       for(int i=0;i<n;i++)
        scanf("%d%d",&x[i],&y[i]);
    
       for(int i=0;i<n;i++){
        for(int j=i+1;j<n;j++){
            //cout<<"~~"<<endl;
            int dx=x[i]-x[j];
            int dy=y[i]-y[j];
            if(dx<0||(dx==0&&dy<0)){
                dx=-dx,dy=-dy;
            }
            v.push_back(make_pair(dx,dy));
        }
       }
       //cout<<"~~"<<endl;
       sort(v.begin(),v.end());
       //printf("size:%d
",v.size());
       LL cnt=0;
       LL ans=0;
       for(int i=0;i<(int)v.size()-1;i++){
           cnt++;
           if(v[i+1]!=v[i]){
             ans+=cnt*(cnt-1)/2;
             cnt=0;
           }
        }
        printf("%I64d
", ans/2);
    }
    return 0;
}