378. Kth Smallest Element in a S

用堆来解决
调用标准库import heapq
先把矩阵最左端一列压入队中
初始化结果
接着循环ｋ次，每次循环把堆顶元素弹出，再压入弹出元素右边的元素（如果存在的话）

class Solution(object):
    def kthSmallest(self, matrix, k):
        """
        :type matrix: List[List[int]]
        :type k: int
        :rtype: int
        """
        heap = [(row[0], i, 0) for i, row in enumerate(matrix)]
        heapq.heapify(heap)
        ret = 0
        for _ in range(k):
            ret, i, j = heapq.heappop(heap)
            if j+1 < len(matrix[0]):
                heapq.heappush(heap, (matrix[i][j+1], i, j+1))
        return ret