https://leetcode.com/problems/rotate-image
You are given an nxn 2D matrix representing an image.
Rotate the image by 90 degrees (clockwise).
Follow up:
Could you do this in-place?
如图所示，我们需要把图上四个点顺时针移动
即
u: matrix[i][j]
r: matrix[j][n- i]
d:matrix[n - i][n - j]
l: matrix[n - j][i]
**// n should be length - 1 in code
method2：
in discuss, they are using geometry, swap up to down, then swap symmetrically
follow up: how to do like rotate one/more steps ??