PattySung

You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise).

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:

Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [[7,4,1],[8,5,2],[9,6,3]]

Example 2:

Input: matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]
Output: [[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]

<解題>

先垂直翻轉（交換第一行和第四行、交換第二行和第三行），再對角線翻轉（交換(1,2) 和 (2,1)，交換 (1,3) 和 (3,1)，交換 (1,4) 和 (4,1)）
或也可以先對角線翻轉，再垂直翻轉


class Solution {
    public void rotate(int[][] matrix) {
        int n = matrix.length;

        // 先對矩陣進行垂直翻轉
        for (int i = 0; i < n / 2; i++) {
            int[] temp = matrix[i]; 
            matrix[i] = matrix[n - 1 - i]; 
            matrix[n - 1 - i] = temp;
        }

        // 再對矩陣進行主對角線翻轉
        for (int i = 0; i < n; i++) {
            for (int j = i + 1; j < n; j++) {
                int temp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = temp;
            }
        }
        //     (0,1)(0,2)(0,3)
        //          (1,2)(1,3)
        //               (2,3)
    }
}

Time: O(N^2) Space: O(1)

Leetcode-48. Rotate Image

<解題>