73. 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.
Example 1:
Input:
[
[1,1,1],
[1,0,1],
[1,1,1]
]
Output:
[
[1,0,1],
[0,0,0],
[1,0,1]
]Example 2:
Input:
[
[0,1,2,0],
[3,4,5,2],
[1,3,1,5]
]
Output:
[
[0,0,0,0],
[0,4,5,0],
[0,3,1,0]
]Follow up:
- 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?
解析
把矩阵中含有0的那一行和列全部置0.
参考答案
自己写的：
class Solution {
public void setZeroes(int[][] matrix) {
Set<Integer> rows = new HashSet<>();
Set<Integer> cols = new HashSet<>();
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[i].length; j++) {
if (matrix[i][j] == 0) {
rows.add(i);
cols.add(j);
}
}
}
for (Integer i : rows) {
for (int j = 0; j < matrix[i].length; j++) {
matrix[i][j] = 0;
}
}
for (Integer i : cols) {
for (int j = 0; j < matrix.length; j++) {
matrix[j][i] = 0;
}
}
}
}一开始没注意题目的意思，这个是一般的解法，题目的意思是不要用额外的空间。
别人写的：
public class Solution {
public void setZeroes(int[][] matrix) {
boolean fr = false,fc = false;
for(int i = 0; i < matrix.length; i++) {
for(int j = 0; j < matrix[0].length; j++) {
if(matrix[i][j] == 0) {
if(i == 0) fr = true;
if(j == 0) fc = true;
matrix[0][j] = 0;
matrix[i][0] = 0;
}
}
}
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;
}
}
}
if(fr) {
for(int j = 0; j < matrix[0].length; j++) {
matrix[0][j] = 0;
}
}
if(fc) {
for(int i = 0; i < matrix.length; i++) {
matrix[i][0] = 0;
}
}
}
}把第0行和第0列留出来记录那些行和哪些列需要置0，用fr和fc来记录第0行和第0列是否需要置零。