73. Set Matrix Zeroes

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

Follow-Up:

Could you devise a constant space solution?

Solution:

class Solution(object):
    def setZeroes(self, matrix):
        """
        :type matrix: List[List[int]]
        :rtype: void Do not return anything, modify matrix in-place instead.
        """
        if not matrix:
            return

        rows = len(matrix)
        cols = len(matrix[0])

        col0 = 1
        for i in xrange(rows):
            if matrix[i][0] == 0:
                col0 = 0
            for j in xrange(1, cols):
                if matrix[i][j] == 0:
                    matrix[i][0] = matrix[0][j] = 0

        for i in xrange(rows - 1, -1, -1):
            for j in xrange(cols - 1, 0, -1):
                if matrix[i][0] == 0 or matrix[0][j] == 0:
                    matrix[i][j] = 0
            if col0 == 0:
                matrix[i][0] = 0

Lessons:

• Use first row to save each column's state.
• Use first column to save each row's state.
• Use a variable to save first column's state.