48 Rotate Image

🔀 Pick One

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:

1 2 3      7 4 1
4 5 6  =>  8 5 2
7 8 9      9 6 3

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

Example 2:

5  1  9  11      15 13 2  5
2  4  8  10  =>  14 3  4  1
13 3  6  7       12 6  8  9
15 14 12 16      16 7  10 11

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]]

Example 3:

Input: matrix = [[1]]
Output: [[1]]

Example 4:

Input: matrix = [[1,2],[3,4]]
Output: [[3,1],[4,2]]

Constraints:

  • n == matrix.length == matrix[i].length
  • 1 <= n <= 20
  • -1000 <= matrix[i][j] <= 1000
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class Solution:
    def rotate(self, matrix: List[List[int]]) -> None:
        """
        Do not return anything, modify matrix in-place instead.
        """
        # transpose: top-right <-> bottom-left
        n = len(matrix)
        for i in range(n):
            for j in range(i + 1, n):
                matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
        # reflect: left <-> right
        for i in range(n):
            for j in range(n // 2):
                matrix[i][j], matrix[i][-j-1] = matrix[i][-j-1], matrix[i][j]
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func rotate(matrix [][]int) {
	n := len(matrix) - 1
	// reverse up to down
	for i := 0; i < len(matrix)/2; i++ {
		for j := 0; j < len(matrix); j++ {
			// swap (i, j), (n-i, j)
			matrix[i][j], matrix[n-i][j] = matrix[n-i][j], matrix[i][j]
		}
	}
	// swap the symmetry
	for i := 0; i < len(matrix); i++ {
		for j := i + 1; j < len(matrix); j++ {
			// swap (i, j), (j, i)
			matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
		}
	}
}