Difficulty:medium
More:【目录】LeetCode Java实现
Description
https://leetcode.com/problems/rotate-image/
You are given an n x n 2D 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 NOTallocate 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]
]
Intuition
1. transpose the matrix
2. flip the matrix horizontally
1 2 3 1 4 7 7 4 1
4 5 6 => 2 5 8 => 8 5 2
7 8 9 3 6 9 9 6 3
Solution
public void rotate(int[][] a) {
int n = a[0].length;
//transpose the matrix
for(int i=0; i<n; i++){
for(int j=i+1; j<n; j++){
int temp = a[i][j];
a[i][j]=a[j][i];
a[j][i]=temp;
}
}
//flip the matrix horizontally
for(int i=0; i<n; i++){
for(int j=0; j<n/2; j++){
int temp = a[i][j];
a[i][j] = a[i][n-1-j];
a[i][n-1-j] = temp;
}
}
}
Complexity
Time complexity : O(N)
Space complexity : O(1)
What I've learned
1. When rotating a matrix, we can transpose the matrix firstly, and then find the law.
More:【目录】LeetCode Java实现