Forest

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <map>
#include <utility>
#include <set>

typedef long long LL;

using namespace std;

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    LL n; cin >> n;
    vector<pair<LL,LL> > points;
    pair<LL,LL> U; cin >> U.first >> U.second;
    
    for(int i = 0; i < n; i++){
        LL x,y; cin >> x >> y;
        points.push_back(make_pair(x,y));
    }
    vector<bool> can_see(n,true);
    vector<pair<LL,LL> > triple(3);
    for(int i = 0; i < n; i++){
        for(int j = i+1; j < n; j++){
            triple[0] = points[i];
            triple[1] = points[j];
            triple[2] = U;
            sort(triple.begin(), triple.end());
            LL x1 = triple[1].first - triple[0].first;
            LL y1 = triple[1].second - triple[0].second;
            LL x2 = triple[2].first - triple[1].first;
            LL y2 = triple[2].second - triple[1].second;
            if(x1*y2 == x2*y1){
                if(triple[0] == U){
                    if(triple[2] == points[i]) can_see[i] = false;
                    else if(triple[2] == points[j]) can_see[j] = false;
                }
                if(triple[2] == U){
                    if(triple[0] == points[i]) can_see[i] = false;
                    else if(triple[0] == points[j]) can_see[j] = false
                        ;                    
                }
            }
        }
    }
    LL ans = 0;
    for(auto b : can_see) if(b) ans++;
    cout << ans << endl;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED