LeetCode - Set Matrix Zeroes
Problem description
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:
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Input:
[
[1,1,1],
[1,0,1],
[1,1,1]
]
Output:
[
[1,0,1],
[0,0,0],
[1,0,1]
]
Example 2:
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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?
Analysis
The general idea is to save information in the first position of each row/column.
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class Solution {
public void setZeroes(int[][] matrix) {
if (matrix == null){
return;
}
int m = matrix.length;
if (m == 0){
return;
}
int n = matrix[0].length;
if (n == 0){
return;
}
boolean col = false;
boolean row = false;
for (int i = 0; i< m ;i++){
if (matrix[i][0] == 0){
col = true;
}
}
for (int i = 0; i< n ;i++){
if (matrix[0][i] == 0){
row = true;
}
}
for (int i = 1; i < m; i++){
for (int j = 1; j< n; j++){
if (matrix[i][j] == 0){
matrix[0][j] = 0;
matrix[i][0] = 0;
}
}
}
for (int i = 1; i < m; i++){
if (matrix[i][0] == 0){
for (int j = 0; j < n; j++){
matrix[i][j] = 0;
}
}
}
for (int j = 1; j < n; j++){
if (matrix[0][j] == 0){
for (int i = 0; i< m; i++){
matrix[i][j] = 0;
}
}
}
if (col){
for (int i = 0; i < m; i++){
matrix[i][0] = 0;
}
}
if (row){
for (int i = 0; i < n; i++){
matrix[0][i] = 0;
}
}
}
}
What to improve
- think and code, be careful about start index.