Set Matrix Zeroes

標籤：

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

Have you met this question in a real interview? YesExample

Given a matrix

[  [1,2],  [0,3]],

return
[
[0,2],
[0,0]
]

Challenge 

Did you use extra space?
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?

Analysis: We are going to use two sets to save all the column numbers and row numbers whose cells need to be set to 0. This will cost O(m + n) space. Another approach is from: http://fisherlei.blogspot.com/2013/01/leetcode-set-matrix-zeroes.html1.先確定第一行和第一列是否需要清零
2.掃描剩下的矩陣元素，如果遇到了0，就將對應的第一行和第一列上的元素賦值為0
3.根據第一行和第一列的資訊，已經可以講剩下的矩陣元素賦值為結果所需的值了
4.根據1中確定的狀態，處理第一行和第一列。


  

 1 public class Solution { 2     /** 3      * @param matrix: 4      *            A list of lists of integers 5      * @return: Void 6      */ 7     public void setZeroes(int[][] matrix) { 8         if (matrix == null || matrix.length == 0 || matrix[0].length == 0) return; 9 10         boolean firstRowZero = false;11         boolean firstColumnZero = false;12 13         // set first row and column zero or not14         for (int i = 0; i < matrix.length; i++) {15             if (matrix[i][0] == 0) {16                 firstColumnZero = true;17                 break;18             }19         }20 21         for (int i = 0; i < matrix[0].length; i++) {22             if (matrix[0][i] == 0) {23                 firstRowZero = true;24                 break;25             }26         }27 28         // mark zeros on first row and column29         for (int i = 1; i < matrix.length; i++) {30             for (int j = 1; j < matrix[0].length; j++) {31                 if (matrix[i][j] == 0) {32                     matrix[i][0] = 0;33                     matrix[0][j] = 0;34                 }35             }36         }37 38         // use mark to set elements39         for (int i = 1; i < matrix.length; i++) {40             for (int j = 1; j < matrix[0].length; j++) {41                 if (matrix[i][0] == 0 || matrix[0][j] == 0) {42                     matrix[i][j] = 0;43                 }44             }45         }46 47         // set first column and row48         if (firstColumnZero) {49             for (int i = 0; i < matrix.length; i++)50                 matrix[i][0] = 0;51         }52 53         if (firstRowZero) {54             for (int i = 0; i < matrix[0].length; i++)55                 matrix[0][i] = 0;56         }57     }58 }

 

 

Set Matrix Zeroes