Array Algorithm: Rotate an 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]]

Example 3:

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

Example 4:

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

Constraints:

matrix.length == n

matrix[i].length == n

1 <= n <= 20

-1000 <= matrix[i][j] <= 1000

Solution

var top = matrix[layer][i];
var right = matrix[i][length - layer];
var bot = matrix[length - layer][length -i];
var left = matrix[length - i][layer];
// top = left
matrix[layer][i] = left;
// right = top
matrix[i][length - layer] = top;
// bot = right
matrix[length - layer][length - i] = right;
// left = bot
matrix[length - i][layer] = bot;
public class Solution {
public void Rotate(int[][] matrix) {
var layers = matrix.Length / 2;
var length = matrix.Length - 1;

for(var layer = 0; layer < layers; layer++){

for(var i = layer; i < length - layer; i++){
var top = matrix[layer][i];
var right = matrix[i][length - layer];
var bot = matrix[length - layer][length -i];
var left = matrix[length - i][layer];

// top = left
matrix[layer][i] = left;
// right = top
matrix[i][length - layer] = top;
// bot = right
matrix[length - layer][length - i] = right;
// left = bot
matrix[length - i][layer] = bot;
}
}
}
}

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