Hi, here's your problem today. This problem was recently asked by Uber:

Given a square 2D matrix (n x n), rotate the matrix by 90 degrees clockwise.

Here's an example and some starting code:

```
def rotate(mat):
# Fill this in.
mat = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
# Looks like
# 1 2 3
# 4 5 6
# 7 8 9
# Looks like
# 7 4 1
# 8 5 2
# 9 6 3
print(rotate(mat))
# [[7, 4, 1], [8, 5, 2], [9, 6, 3]]
```