LeetCode 73. Set Matrix Zeroes

Description

https://leetcode.com/problems/set-matrix-zeroes/

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?

Explanation

If any cell of the matrix has a zero we can record its row and column number.

Python Solution

class Solution:
    def setZeroes(self, matrix: List[List[int]]) -> None:
        """
        Do not return anything, modify matrix in-place instead.
        """
                
        zero_rows = set()
        zero_columns = set()
        
        for i in range(0, len(matrix)):
            for j in range(0, len(matrix[0])):                
                if matrix[i][j] == 0:
                    zero_rows.add(i)
                    zero_columns.add(j)                    
                    
        
        for i in range(0, len(matrix)):
            for j in range(0, len(matrix[0])):                
                if i in zero_rows or j in zero_columns:
                    matrix[i][j] = 0
            
  • Time complexity: O(MN).
  • Space complexity: O(M + N).

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