Count Number of Triplets

    1 
    2#include <bits/stdc++.h>
    3 
    4using namespace std;
    5 
    6struct triplet{
    7    int x, y, z;
    8    triplet(int a, int b, int c) : x(a), y(b), z(c){}
    9    bool operator<(const triplet &o) const{
    10        if(x != o.x) return x < o.x;
    11        if(y != o.y) return y < o.y;
    12        return z < o.z;
    13    }
    14};
    15int main(){
    16    ios::sync_with_stdio(0);
    17    cin.tie(0);
    18    int n;
    19    cin >> n;
    20    vector<int> v(n);
    21    for(int i = 0; i < n; i++){
    22        cin >> v[i];
    23    }
    24    sort(v.begin(), v.end());
    25    set<triplet> s;
    26    int cnt = 0;
    27    for(int i = 0; i < v.size(); i++){
    28        for(int j = i + 1; j < v.size(); j++){
    29            int sum = v[i] + v[j];
    30            auto pos = lower_bound(v.begin(), v.end(), sum);
    31            if(pos == v.end() || v[pos - v.begin()] != sum) continue;
    32            if(pos != v.end() && !s.count({v[i], v[j], sum})){
    33                cnt++;
    34                s.insert({v[i], v[j], sum});
    35            }
    36        }
    37    }
    38    cout << (cnt == 0 ? -1 : cnt) << '\n';
    39    return 0;
    40}