問題

給定一個n*n的矩陣，要將矩陣順時針旋轉90度，且必須直接修改輸入的矩陣，不可以另外宣告新的矩陣．

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).
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]]

Constraints:

n == matrix.length == matrix[i].length
1 <= n <= 20
-1000 <= matrix[i][j] <= 1000

我的答案

先依對角線翻轉，在左右翻轉．如下圖所示：

class Solution:
    def rotate(self, matrix: List[List[int]]) -> None:
        """
        Do not return anything, modify matrix in-place instead.
        """
        n = len(matrix)

        # transpose
        for i in range(n-1):
            for j in range(i+1,n):
                matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]

        # refelct
        for i in range(n):
            for j in range(n//2):
                matrix[i][j], matrix[i][n-j-1] = matrix[i][n-j-1], matrix[i][j]

Go

func rotate(matrix [][]int)  {
    n := len(matrix)
    
    for i := 0 ; i < n-1 ; i++ {
        for j := i+1 ; j < n ; j++ {
            matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
        }
    }
    
    for i := 0 ; i < n ; i++ {
        for j := 0; j < n/2; j++ {
            matrix[i][j], matrix[i][n-j-1] = matrix[i][n-j-1], matrix[i][j]
        }
    }
}