Codeforces Round #558 (Div. 2) C. Power Transmission(思维 map+set)

#include <bits/stdc++.h>
#define ll long long
#define inf 1e9
using namespace std;
typedef pair<double,double> p;
vector<pair<double,double> > v;
unordered_map<double ,set<double> > ma;
int n;

int main()
{
  scanf("%d", &n);
  for(int i=0;i<n;i++){
    double x, y;
    scanf("%lf%lf",&x, &y);
    v.push_back(p(x, y));
  }
  for(int i=0;i<v.size();i++){
    for(int j=i+1;j<v.size();j++){
      double xx, yy;
      if(v[i].first == v[j].first){
        xx = inf;
        yy = v[i].first;
      }
      else{
        xx = ((double)v[i].second - v[j].second) / (v[i].first - v[j].first);
        yy = v[i].second - xx * v[i].first;
      }
      ma[xx].insert(yy);
    }
  }
  ll cnt = 0;
  for(auto p : ma){
    cnt += p.second.size();
  }
  ll ans = 0;
  for(auto p : ma){
    ans += (cnt - p.second.size()) * p.second.size();
  }
  printf("%lld\n", ans / 2);
  return 0;
}