73. 矩阵置零

思路

先获取元素为0所在的行和列，用集合rows 存放元素为0的行，集合cols存放元素为0的列

遍历矩阵，遇到的行、列为0元素所在行、列，将对应行、列的元素设置为0

ps：用集合可以去重、用in时时间复杂度O(1)

class Solution(object):
    def setZeroes(self, matrix):
        """
        :type matrix: List[List[int]]
        :rtype: None Do not return anything, modify matrix in-place instead.
        """
        rows=set()
        cols=set()
        for i in range(len(matrix)):
            for j in range(len(matrix[i])):
                if matrix[i][j]==0:
                    rows.add(i)
                    cols.add(j)
        for i in range(len(matrix)):
            for j in range(len(matrix[i])):
                if i in rows:
                    matrix[i][j]=0
                else:
                    if j in cols:
                        matrix[i][j]=0
        return matrix