Kth smallest across lists (K-way Merge Sub-pattern)

// Kth smallest in a matrix whose rows (and columns) are sorted: pop k times.
int kthSmallest(vector<vector<int>>& matrix, int k) {
    int n = matrix.size();
    priority_queue<tuple<int, int, int>, vector<tuple<int, int, int>>, greater<>> pq;  // (value, row, col)
    for (int r = 0; r < min(n, k); r++) pq.push({matrix[r][0], r, 0});  // rows past k can't matter
    for (int pops = 1; pops < k; pops++) {                  // throw away the k-1 smallest
        auto [v, r, c] = pq.top(); pq.pop();
        if (c + 1 < (int)matrix[r].size()) pq.push({matrix[r][c + 1], r, c + 1});
    }
    return get<0>(pq.top());                                // the kth pop
}