leetcode 73 Set Matrix zeros
z
 2018-03-05 11:10:10  

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

Follow up:

Did you use extra space?
A straight forward solution using O(m**n) 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?

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class Solution {
    public void setZeroes(int[][] matrix) {
        if(matrix == null || matrix.length == 0) return ;
        boolean firstRow = false;
        boolean firstCol = false;
        
        for(int i=0; i<matrix.length; i++){
            for(int j=0; j<matrix[0].length; j++){
                if(matrix[i][j] == 0){
                    if(i == 0) firstRow = true;
                    if(j == 0) firstCol = true;
                    
                    matrix[0][j] = 0;
                    matrix[i][0] = 0;
                }
            }
        }
        for(int i=1; i<matrix.length; i++){
            for(int j=1; j<matrix[0].length; j++){
                if(matrix[i][0] == 0 || matrix[0][j] == 0){
                    matrix[i][j] = 0;
                }
            }
        }
        
        if(firstRow){
            for(int i=0; i<matrix[0].length; i++){
                matrix[0][i] = 0;
            }
        }
        if(firstCol){
            for(int i=0; i<matrix.length; i++){
                matrix[i][0] = 0;
            }
        }
        return ;
    }
}
  • 如果matrix[i][j] == 0， 则将第i行的第一个元素和第j列的第一个元素置0
  • 扫描第一行和第一列，将对应的行和列置0
  • 注意第一行和第一列需要特殊处理。