48. Rotate Image

Description

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 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]
]

Solution

Iteration

由外至内一圈一圈的、四个为一组的变换。注意边界条件。

class Solution {
    public void rotate(int[][] matrix) {
        int n = matrix.length;
        
        for (int i = 0; i < n / 2; ++i) {
            for (int j = i; j < n - i - 1; ++j) {
                // 1 = matrix[i][j];
                // 2 = matrix[j][n - i - 1];
                // 3 = matrix[n - i - 1][n - j - 1];
                // 4 = matrix[n - j - 1][i];
                int tmp = matrix[n - j - 1][i];
                matrix[n - j - 1][i] = matrix[n - i - 1][n - j - 1];
                matrix[n - i - 1][n - j - 1] = matrix[j][n - i - 1];
                matrix[j][n - i - 1] = matrix[i][j];
                matrix[i][j] = tmp;
            }
        }
    }
}