LeetCode(073) Set Matrix Zeroes (Java)

题目如下：

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(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?

题目分析:

1. 空间要求是in place。 用矩阵第一行记录哪些列要被设定为0，要矩阵第一行记录那系列要被设定为0，然后用一个变量记录第一行本身是否要被设定为0，同理用另外一个变量记录第一列本身是否要被设定为0.

2.  java的boolean型不能赋值给int型，这和C++是不一样。所以下面这行错误

//错误代码 //
int firstRowHasZero = false

3. 2D array的行数和列数如何获得呢?

行数，比如有matrix.length,那么列数呢？题目中说了这是个矩阵，所以必然是matrix[0].length.

可是，要注意的是，array和2Darray是用object来实现的，所以每行的长度不一定相同。看看下面这个解释:

Java allows you to create "ragged arrays" where each "row" has different lengths.
[[], [0], [0, 0], [0, 0, 0], [0, 0, 0, 0]]
[[], [0], [0, 0], [0, 0, 0], [0, 0, 0, 0]]
If you know you have a square array, you can use your code modified to protect against an empty array like this:
if (row > 0) col = test[0].length;

我的代码:

//305ms
public class Solution {
    public void setZeroes(int[][] matrix) {
        //int firstRowHasZero = false; // error: incompatible types: boolean cannot be converted to int
        boolean firstRowHasZero = false;
        boolean firstColHasZero = false;
        if (matrix.length == 0) return;
        for (int i = 0; i < matrix[0].length; ++i) {
            if (matrix[0][i] == 0) {
                firstRowHasZero = true;
                break;
            }
        }
        for (int i = 0; i < matrix.length; ++i) {
            if (matrix[i][0] == 0) {
                firstColHasZero = true;
                break;
            }
        }
        for (int i = 1; i < matrix.length; ++i) {
            for (int j = 1; j < matrix[i].length; ++j) {
                if (matrix[i][j] == 0) {
                    matrix[0][j] = 0;
                    matrix[i][0] = 0;
                }
            }
        }
        for (int i = 1; i < matrix.length; ++i) {
            for (int j = 1; j < matrix[i].length; ++j) {
                if (matrix[0][j] == 0 || matrix[i][0] == 0) {
                    matrix[i][j] = 0;
                }
            }
        }
        if (firstRowHasZero) {
            for (int i = 0; i < matrix[0].length; ++i) {
                matrix[0][i] = 0;
            }
        }
        if (firstColHasZero) {
            for (int i = 0; i < matrix.length; ++i) {
                matrix[i][0] = 0;
            }
        }        
    }
}