Rotate Image

Question: You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Note:

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:

Given input matrix = 
[
  [1,2,3],
  [4,5,6],
  [7,8,9]
],
rotate the input matrix in-place such that it becomes:
[
  [7,4,1],
  [8,5,2],
  [9,6,3]
]

You may view the full question here.

Approach 1: The discussion forum had some brilliant ideas. One approach is —

• Reverse the rows of the matrix — top-down manner
• Swap the elements of the matrix diagonally, with the diagonal starting from the top left corner to the bottom right corner of the matrix

Here’s the code —

//Approach 1:
//Runtime: 0ms
//Memory usage: 35.5MB
class Solution {
    public void rotate(int[][] matrix) {
        int len = matrix.length;
        //Reverse
        for(int r = 0; r<len/2; r++){
            for(int c = 0; c<matrix[0].length; c++){
                int temp = matrix[r][c];
                matrix[r][c] = matrix[len-r-1][c];
                matrix[len-r-1][c] = temp;
            }
        }
        //Swap
        for(int r = 0; r<len; r++){
            for(int c = r+1; c<matrix[0].length; c++){
                int temp = matrix[r][c];
                matrix[r][c] = matrix[c][r];
                matrix[c][r] = temp;
            }
        }
    }
}

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Cheers & Chao!