378. 有序矩阵中第K小的元素

class Solution {
    public int kthSmallest(int[][] matrix, int k) {
        PriorityQueue<Integer> priorityQueue = new PriorityQueue<>();
        for (int[] arr1: matrix){
            for(int num: arr1){
                priorityQueue.add(num);
            }
        }
        int rtn = 0;

        while (k-- > 0){
            rtn = priorityQueue.poll();

        }

        return rtn;
    }
}