048-rotate-image
Question
https://leetcode.com/problems/rotate-image/description/
You are given annxn2D matrix representing an image.
Rotate the image by 90 degrees (clockwise).
Note:
You have to rotate the image in-place, which means you have to modify the input 2D matrix directly.DO NOT allocate another 2D matrix and do the rotation.
Example 1:
Given input matrix =
[
[1,2,3],
[4,5,6],
[7,8,9]
],
rotate the input matrix in-place such that it becomes:
[
[7,4,1],
[8,5,2],
[9,6,3]
]
Example 2:
Given input matrix =
[
[ 5, 1, 9,11],
[ 2, 4, 8,10],
[13, 3, 6, 7],
[15,14,12,16]
],
rotate the input matrix in-place such that it becomes:
[
[15,13, 2, 5],
[14, 3, 4, 1],
[12, 6, 8, 9],
[16, 7,10,11]
]
Thought Process
- Follow the requirement, and swap one element at the time
- Take the top left corner element put it in right top, then right top to bottom right, then bottom left
- Then we proceed to the next element
- We do this layer by layer until we reach to the core
- Trick
- Flip upside down (flip across the middle)
- Flip diagonally from top left to bottom right
Solution
class Solution {
public void rotate(int[][] matrix) {
int n = matrix.length;
// we start from layer 0, and going inward
for (int l = 0; l < n / 2; l++) {
for (int c = l; c < n - 1 - l; c++) {
int tmp = matrix[l][c];
matrix[l][c] = matrix[n - 1 - c][l];
matrix[n - 1 - c][l] = matrix[n - 1 - l][n - 1 - c];
matrix[n - 1 - l][n - 1 - c] = matrix[c][n - 1 - l];
matrix[c][n - 1 - l] = tmp;
}
}
}
}
class Solution {
public void rotate(int[][] matrix) {
int n = matrix.length;
int tmp = 0;
int[] tmpA = null;
//Flip up and down, horizontal accross the middle line
for(int i =0; i < n/2; i++){
tmpA = matrix[i];
matrix[i] = matrix[n-i-1];
matrix[n-i-1] = tmpA;
}
//Flip diagonally from top left to bottom right
for(int i = 0; i < n; i++){
for(int j = i + 1; j < n; j++){
tmp = matrix[i][j];
matrix[i][j] = matrix[j][i];
matrix[j][i] = tmp;
}
}
}
}