Leetcode 73. Set Matrix Zeroes

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

 

Medium

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?

---

• 把首行和首列作为标记位，如果某行某列为0，则相应标记为0。之后再扫描一遍，如果标记位为0，则相应行列都改为0。
• 需要注意把首行首列提前检查，避免混淆。

class Solution:
    def setZeroes(self, matrix: List[List[int]]) -> None:
        """
        Do not return anything, modify matrix in-place instead.
        """
        first_row, first_col = False, False
        
        # check the first row and column first
        for i in range( len( matrix ) ):
            if matrix[i][0] == 0:
                first_col = True;
                break;
                
        for i in range( len( matrix[0] ) ):
            if matrix[0][i] == 0:
                first_row = True;
                break;
                
        # use the first row and column as marker
        for i in range( 1, len( matrix ) ):
            for j in range( 1, len( matrix[0] ) ):
                if matrix[i][j] == 0:
                    matrix[i][0], matrix[0][j] = 0, 0;
        
        for i in range( 1, len( matrix ) ):
            for j in range( 1, len( matrix[0] ) ):
                if matrix[i][0] == 0 or matrix[0][j] == 0:
                    matrix[i][j] = 0
                    
        if first_row:
            for i in range( len( matrix[0] ) ):
                matrix[0][i] = 0;
                
        if first_col:
            for i in range( len( matrix ) ):
                matrix[i][0] = 0;