Rotate Image

Problem

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

Solution

/**
 * @param {number[][]} matrix
 * @return {void} Do not return anything, modify matrix in-place instead.
 */
var rotate = function(matrix) {
    // transpose matrix
    for (let i = 0; i < matrix.length; i++) {
        for (let j = i; j < matrix[i].length; j++) {
            const temp = matrix[i][j];
            matrix[i][j] = matrix[j][i];
            matrix[j][i] = temp;
        }
    }

    // reflect matrix
    for (let i = 0; i < matrix.length; i++) {
        for (let j = 0; j < Math.floor(matrix[i].length / 2); j++) {
            const temp = matrix[i][j];
            matrix[i][j] = matrix[i][matrix[i].length - j - 1];
            matrix[i][matrix[i].length - j - 1] = temp;
        }
    }

    return matrix;
};

To rotate a matrix clockwise, we can first transpose the matrix and then reflect it.