Problem
You are given an n x n 2D matrix representing an image.
Rotate the image by 90 degrees (clockwise).
Follow up:
Could you do this in-place?
Could you do this in-place?
Analysis
对于一个n*n的矩阵，可以像剥洋葱一样对每一层进行rotate，一共有n/2层
对于每一层，采用 matrix[i][j] = matrix[n-1-j][i] 进行 in-place的rotation
时间复杂度: O(n^2)
空间复杂度: O(n^2)