【数三角——枚举，哈希】

题目

代码

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<int, int>;

#define x first
#define y second

const int N = 2010;

pll a[N];
int main()
{
    int n;
    scanf("%d", &n);

    map<pll, int> cnt;
    for (int i = 1; i <= n; i++)
    {
        scanf("%d%d", &a[i].x, &a[i].y);
        cnt[{a[i].x, a[i].y}]++;
    }

    ll ans = 0;
    for (int i = 1; i <= n; i++)
    {
        map<ll, vector<int>> lenmp;
        for (int j = 1; j <= n; j++)
        {
            ll len = 1ll * (a[i].x - a[j].x) * (a[i].x - a[j].x) + 1ll * (a[i].y - a[j].y) * (a[i].y - a[j].y);
            if (len)
                lenmp[len].push_back(j);
        }

        for (auto &u : lenmp)
        {
            vector<int> &id = u.y;
            int sz = id.size();
            if (sz < 2)
                continue;
            ans += sz * (sz - 1) / 2;

            int del = 0;
            for (int j = 0; j < id.size(); j++)
            {
              int u = id[j];
              int cx = a[i].x, cy = a[i].y;
              int x = a[u].x, y = a[u].y;
              int px = 2 * cx - x, py = 2 * cy - y;
              del += cnt[{px, py}];
            }

            ans -= del / 2;
        }
    }

    printf("%lld", ans);
}