LeetCode 73. Set Matrix Zeroes（矩阵清零）

原题网址：https://leetcode.com/problems/set-matrix-zeroes/

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?

方法一：使用行和列哈希。

public class Solution {
public void setZeroes(int[][] matrix) {
boolean[] row = new boolean[matrix.length];
boolean[] col = new boolean[matrix[0].length];
for(int i=0; i<matrix.length; i++) {
for(int j=0; j<matrix[i].length; j++) {
if (matrix[i][j]==0) {
row[i] = true;
col[j] = true;
}
}
}
for(int i=0; i<matrix.length; i++) {
if (row[i]) {
for(int j=0; j<matrix[i].length; j++) matrix[i][j] = 0;
}
}
for(int j=0; j<matrix[0].length; j++) {
if (col[j]) {
for(int i=0; i<matrix.length; i++) matrix[i][j] = 0;
}
}
}
}

方法二：将哈希映射到第一行和第一列，注意顺序问题。

public class Solution {
public void setZeroes(int[][] matrix) {
boolean top = false;
for(int j=0; j<matrix[0].length; j++) {
if (matrix[0][j] == 0) {
top = true;
break;
}
}

boolean left = false;
for(int i=0; i<matrix.length; i++) {
if (matrix[i][0] == 0) {
left = 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[i][0] = 0;
matrix[0][j] = 0;
}
}
}
for(int i=1; i<matrix.length; i++) {
for(int j=1; j<matrix[i].length; j++) {
if (matrix[i][0] == 0 || matrix[0][j] == 0) matrix[i][j] = 0;
}
}
if (top) Arrays.fill(matrix[0], 0);
if (left) {
for(int i=0; i<matrix.length; i++) matrix[i][0] = 0;
}
}
}