rotate-image.js

/**
 * Rotate Image
 *
 * 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]
 * ]
 * Example 2:
 *
 * Given input matrix =
 * [
 *   [ 5, 1, 9,11],
 *   [ 2, 4, 8,10],
 *   [13, 3, 6, 7],
 *   [15,14,12,16]
 * ],
 *
 * rotate the input matrix in-place such that it becomes:
 * [
 *   [15,13, 2, 5],
 *   [14, 3, 4, 1],
 *   [12, 6, 8, 9],
 *   [16, 7,10,11]
 * ]
 */

/*
 * clockwise rotate
 * first reverse up to down, then swap the symmetry 
 * 1 2 3     7 8 9     7 4 1
 * 4 5 6  => 4 5 6  => 8 5 2
 * 7 8 9     1 2 3     9 6 3
*/
const rotate = matrix => {
  matrix.reverse();

  for (let i = 0; i < matrix.length; i++) {
    for (let j = i + 1; j < matrix[i].length; j++) {
      [matrix[i][j], matrix[j][i]] = [matrix[j][i], matrix[i][j]];
    }
  }
};

/*
 * anticlockwise rotate
 * first reverse left to right, then swap the symmetry
 * 1 2 3     3 2 1     3 6 9
 * 4 5 6  => 6 5 4  => 2 5 8
 * 7 8 9     9 8 7     1 4 7
*/
const anti_rotate = matrix => {
  for (let row of matrix) {
    row.reverse();
  }

  for (let i = 0; i < matrix.length; i++) {
    for (let j = i + 1; j < matrix[i].length; j++) {
      [matrix[i][j], matrix[j][i]] = [matrix[j][i], matrix[i][j]];
    }
  }
};

export { rotate, anti_rotate };