题目

You are given an n x n 2D matrix representing an image.
Rotate the image by 90 degrees (clockwise).

一个n*n的二维数组，代表一张图片，将这张图片顺时针旋转90°后，返回旋转后的二维数组

• 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]
]

• 思路
  原数组取其转置矩阵，然后把每行的数字翻转可得到结果

1 2 3　　　 　　 1 4 7　　　　　 7 4 1

4 5 6　　-->　　 2 5 8　　 --> 　 8 5 2

7 8 9 　　　 　　3 6 9　　　　 9 6 3

• 解法

function rotateImage(matrix){
  const len = matrix.length
  for(var i = 0; i < len; i++){
    for(var j = i + 1; j < len; j++){
      let temp = arr[i][j]
      arr[i][j] = arr[j][i]
      arr[j][i] = temp
    }
  }
  for(i = 0; i < len; i++){
    matrix[i].reverse()
  }
  return matrix
}