Educational Codeforces Round 11-D. Number of Parallelograms

用两条边平行且相等，判断平行四边形。
把所有边转换成向量。mulitest 记录每种向量的个数，在统计

#include <bits/stdc++.h>
#define maxn 100005
#define INF 1e9
using namespace std;
typedef long long ll;

struct Node{
    Node(){
    }
    Node(int a, int b){
        x = a;
        y = b;
    }
    int x, y;
    ll len;
    friend bool operator < (const Node &p1, const Node &p2){
        if(p1.x == p2.x && p1.y == p2.y)
          return p1.len < p2.len;
        if(p1.x == p2.x)
          return p1.y < p2.y;
        return p1.x < p2.x;
    }
};
int x[2005], y[2005];
multiset<Node> m;
int main(){

   // freopen("in.txt", "r", stdin);
    int n;

    while(cin >> n){

        m.clear();
        for(int i = 0; i < n; i++){
            scanf("%d%d", x+i, y+i);
        }
        ll cnt = 0;

        for(int i = 1; i < n; i++){
          for(int j = 0; j < i; j++){
            int h1 = x[i] - x[j], h2 = y[i] - y[j];
            Node p;
            if(h1 == 0){
             p = Node(h1, abs(h2));
            }
            else{
              ll d = h1 * h2;
              p = Node(abs(h1), d / abs(h1));
             }
            p.len = ((ll)h1 * h1 + (ll)h2 * h2);
            cnt += m.count(p);
            m.insert(p);
          }
        }
       cout << cnt / 2 << endl;
    }
    return 0;
}