*Set Matrix Zeroes

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：

解法一：用额外空间

public void setZeroes(int[][] matrix) {
        int m = matrix.length;
        int n = matrix[0].length;
        
        if(m==0||n==0)
            return;
        int[] flagr = new int[m];
        int[] flagc = new int[n];
        
        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;
                }
            }
        }
    }

解法二:不需要额外空间

public void setZeroes(int[][] matrix) {
    int rownum = matrix.length;
    if (rownum == 0)  return;
    int colnum = matrix[0].length;
    if (colnum == 0)  return;

    boolean hasZeroFirstRow = false, hasZeroFirstColumn = false;

    // Does first row have zero?
    for (int j = 0; j < colnum; ++j) {
        if (matrix[0][j] == 0) {
            hasZeroFirstRow = true;
            break;
        }
    }

    // Does first column have zero?
    for (int i = 0; i < rownum; ++i) {
        if (matrix[i][0] == 0) {
            hasZeroFirstColumn = true;
            break;
        }
    }

    // find zeroes and store the info in first row and column
    for (int i = 1; i < matrix.length; ++i) {
        for (int j = 1; j < matrix[0].length; ++j) {
            if (matrix[i][j] == 0) {
                matrix[i][0] = 0;
                matrix[0][j] = 0;
            }
        }
    }

    // set zeroes except the first row and column
    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;
        }
    }

    // set zeroes for first row and column if needed
    if (hasZeroFirstRow) {
        for (int j = 0; j < colnum; ++j) {
            matrix[0][j] = 0;
        }
    }
    if (hasZeroFirstColumn) {
        for (int i = 0; i < rownum; ++i) {
            matrix[i][0] = 0;
        }
    }
}

reference：http://www.cnblogs.com/springfor/p/3888003.html