73 - Set Matrix Zeroes

2020/08/13

leetcode

Problem

Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in-place.

Example 1:

Input:
[
  [1,1,1],
  [1,0,1],
  [1,1,1]
]
Output:
[
  [1,0,1],
  [0,0,0],
  [1,0,1]
]

Example 2:

Input:
[
  [0,1,2,0],
  [3,4,5,2],
  [1,3,1,5]
]
Output:
[
  [0,0,0,0],
  [0,4,5,0],
  [0,3,1,0]
]
Follow up:

A straight forward solution using O(mn) space is probably a bad idea.
A simple improvement uses O(m + n) space, but still not the best solution.
Could you devise a constant space solution?

Solution

class Solution:
    def setZeroes(self, matrix: List[List[int]]) -> None:
        """
        Do not return anything, modify matrix in-place instead.
        """
        col_zero = False

        for i in range(len(matrix)):
            if matrix[i][0] == 0:
                col_zero = True
            for j in range(1, len(matrix[0])):
                if matrix[i][j] == 0:
                    matrix[0][j] = matrix[i][0] = 0

        for i in range(1, len(matrix)):
            for j in range(1, len(matrix[0])):
                if not matrix[0][j] or not matrix[i][0]:
                    matrix[i][j] = 0

        if matrix[0][0] == 0:
            for j in range(len(matrix[0])):
                matrix[0][j] = 0

        if col_zero:
            for i in range(len(matrix)):
                matrix[i][0] = 0