【相反数】暴力即可

暴力遍历+一点点剪枝，没什么好说的

#include<iostream>
#include<vector>
using namespace std;int N;int main(){cin >> N;vector<int> num;vector<bool> vis(N+3,false);for(int i = 0;i < N;i++){int temp;cin >> temp;num.push_back(temp);}int sum = 0;for(int i = 0;i < N;i++){if(!vis[i]){for(int j = i+1;j<N;j++){if(!vis[j]){if(num[i]+num[j] == 0){sum++;vis[j] = true;break;}}}}vis[i] = true;}cout << sum;
}