Leetcode: https://leetcode.com/problems/rotate-image/description/

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]

- (x, y), the affected point is

**(y, n – x – 1), (n – x – 1, n – y – 1), (n – y – 1, x)**, “n” is the size of the matrix.

**The rotate function= **

**(x, y) -> (y, n – x – 1) -> (n – x – 1, n – y – 1) -> (n – y – 1, x) -> (x, y). **
- For the i th line, we start
**from (i, i) **( !! in for loop **j = i** )** , and ends at (i, n – 2 – i)**. ** **
- We only need process half of them.
**n/2**

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