N max pair combinations

• Time : O(nlogn)
• Space : O(n)

struct PairHash {inline std::size_t operator()(const std::pair<int, int> &v) const { return v.first * 31 + v.second; }};
typedef unordered_set<pair<long long, long long>, PairHash> uspll;

vector<int> Solution::solve(vector<int> &A, vector<int> &B) {
    sort(begin(A), end(A));
    sort(begin(B), end(B));
    vector<int> res;
    priority_queue<array<int,3>> Q;
    int n = A.size();
    uspll us;
    us.insert({n-1,n-1});
    Q.push({A[n-1] + B[n-1], n-1, n-1});
    
    while(res.size() < n) {
        auto [sum, i, j] = Q.top(); Q.pop();
        res.push_back(sum);
        if(i - 1 >= 0 and !us.count({i-1,j})) {
            us.insert({i-1,j});
            Q.push({A[i-1] + B[j], i-1, j});
        }
        if(j - 1 >= 0 and !us.count({i,j-1})) {
            us.insert({i,j-1});
            Q.push({A[i] + B[j-1], i, j-1});
        }
    }
    return res;
}