2491. Divide Players Into Teams of Equal Skill

You are given a positive integer array skill of even length n where skill[i] denotes the skill of the ith player. Divide the players into n / 2 teams of size 2 such that the total skill of each team is equal.

The chemistry of a team is equal to the product of the skills of the players on that team.

Return the sum of the chemistry of all the teams, or return -1 if there is no way to divide the players into teams such that the total skill of each team is equal.

class Solution {
public:
    long long dividePlayers(vector<int>& A) {
        if(A.size() % 2 == 1) return -1;
        long long sum = 0, n = A.size(), div = n / 2;
        unordered_map<int, int> freq;
        for(auto a : A) {
            sum += a;
            freq[a] += 1;
        }
        if(sum % div) return -1;
        long long target = sum / div;
        long long res = 0;
        for(auto a : A) {
            if(!freq.count(a)) continue;
            long long o = target - a;
            if(!freq.count(o)) return -1;
            res += o * a;
            if(--freq[o] == 0) freq.erase(o);
            if(!freq.count(a)) return -1;
            if(--freq[a] == 0) freq.erase(a);
        }
        if(freq.size()) return -1;
        return res;
    }
};