Merge K sorted arrays!

• Time : $O(nmlogn)$
• Space : $O(n)$

vector<int> Solution::solve(vector<vector<int> > &A) {
    priority_queue<array<int,3>, vector<array<int,3>>, greater<array<int,3>>> q;
    for(int i = 0; i < A.size(); i++) q.push({A[i][0], i, 0});
    vector<int> res;
    while(q.size()) {
        auto [v,r,c] = q.top(); q.pop();
        res.push_back(v);
        if(A[r].size() != c + 1) q.push({A[r][c+1],r,c+1});
    }
    
    return res;
}