888. Fair Candy Swap.cpp

class Solution {
public:
    vector<int> fairCandySwap(vector<int>& A, vector<int>& B) {
        int sumA(0), sumB(0), half;
        unordered_set<int>s;
        for(auto& x: A) sumA += x;
        for(auto& x: B) sumB += x, s.insert(x);
        half = (sumA + sumB) / 2;
        for(auto& x: A) if(s.count(half - (sumA - x))) return {x, half - (sumA - x)};
    }
};