public int kthSmallest(int[][] matrix, int k) {

PriorityQueue<int[]> priorityQueue = new PriorityQueue<>(new Comparator<int[]>() {

@Override

public int compare(int[] o1, int[] o2) {

return o1[0] - o2[0];

}

});

int n = matrix.length;

for (int i = 0; i < n; i++) {

priorityQueue.offer(new int[]{matrix[i][0], i, 0});

}

for (int i = 0; i < k - 1; i++) {

int[] now = priorityQueue.poll();

if (now[2] != n - 1) {

priorityQueue.offer(new int[]{matrix[now[1]][now[2] + 1], now[1], now[2] + 1});

}

}

return priorityQueue.poll()[0];

}