73. 矩阵置零

可以选择遍历两遍matrix，一遍记录哪些行哪些列需要置0，一遍置0

O(1)空间复杂度的做法：

首先设置两个boolean型变量记录第0行和第0列是否需要置0

然后用第0行和第0列记录需要置0的行和列

class Solution {
    public void setZeroes(int[][] matrix) {
        int m = matrix.length, n = matrix[0].length;
        boolean row0 = false, col0 = false;
        for (int j = 0; j < n; j++) {
            if (matrix[0][j] == 0) row0 = true;
        }
        for (int i = 0; i < m; i++) {
            if (matrix[i][0] == 0) col0 = true;
        }
        for (int i = 1; i < m; i++) {
            for (int j = 1; j < n; j++) {
                if (matrix[i][j] == 0) matrix[0][j] = matrix[i][0] = 0;
            }
        }
        // set 0
        for (int i = 1; i < m; i++) {
            for (int j = 1; j < n; j++) {
                if (matrix[i][0] == 0 || matrix[0][j] == 0) matrix[i][j] = 0;
            }
        }
        if (row0) {
            for (int j = 0; j < n; j++) {
                matrix[0][j] = 0;
            }
        }
        if (col0) {
            for (int i = 0; i < m; i++) {
                matrix[i][0] = 0;
            }
        }
    }
}