Set Matrix Zeroes leetcode java

题目：

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

click to show follow up.

Follow up:

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?

题解：

这道题是CC150 1.7上面的原题。可以看上面详尽的解释，我就不写了。

代码如下：

 1     public void setZeroes(int[][] matrix) {

 2         int m = matrix.length;

 3         int n = matrix[0].length;

 4         

 5         if(m==0||n==0)

 6             return;

 7         int[] flagr = new int[m];

 8         int[] flagc = new int[n];

 9         

         for(int i=0;i<m;i++){

             for(int j=0;j<n;j++){

                 if(matrix[i][j]==0){

                     flagr[i]= 1; 

                     flagc[j]= 1;

                 }

             }

         }

         

         for(int i=0;i<m;i++){

             for(int j=0;j<n;j++){

                 if(flagr[i]==1||flagc[j]==1){

                     matrix[i][j]=0;

                 }

             }

         }

     }

另一种方法是不需要额外空间的，代码来自discuss：

 1 public void setZeroes(int[][] matrix) {
 2     int rownum = matrix.length;
 3     if (rownum == 0)  return;
 4     int colnum = matrix[0].length;
 5     if (colnum == 0)  return;
 6 
 7     boolean hasZeroFirstRow = false, hasZeroFirstColumn = false;
 8 
 9     // Does first row have zero?
10     for (int j = 0; j < colnum; ++j) {
11         if (matrix[0][j] == 0) {
12             hasZeroFirstRow = true;
13             break;
14         }
15     }
16 
17     // Does first column have zero?
18     for (int i = 0; i < rownum; ++i) {
19         if (matrix[i][0] == 0) {
20             hasZeroFirstColumn = true;
21             break;
22         }
23     }
24 
25     // find zeroes and store the info in first row and column
26     for (int i = 1; i < matrix.length; ++i) {
27         for (int j = 1; j < matrix[0].length; ++j) {
28             if (matrix[i][j] == 0) {
29                 matrix[i][0] = 0;
30                 matrix[0][j] = 0;
31             }
32         }
33     }
34 
35     // set zeroes except the first row and column
36     for (int i = 1; i < matrix.length; ++i) {
37         for (int j = 1; j < matrix[0].length; ++j) {
38             if (matrix[i][0] == 0 || matrix[0][j] == 0)  matrix[i][j] = 0;
39         }
40     }
41 
42     // set zeroes for first row and column if needed
43     if (hasZeroFirstRow) {
44         for (int j = 0; j < colnum; ++j) {
45             matrix[0][j] = 0;
46         }
47     }
48     if (hasZeroFirstColumn) {
49         for (int i = 0; i < rownum; ++i) {
50             matrix[i][0] = 0;
51         }
52     }
53 }