Create an identity matrix of the specified size( >= 0).
Some examples:
```
(1)  =>  [[1]]
(2) => [ [1,0],
         [0,1] ]
       [ [1,0,0,0,0],
         [0,1,0,0,0],
(5) =>   [0,0,1,0,0],
         [0,0,0,1,0],
         [0,0,0,0,1] ]   
```
def get_matrix(n):
    return [[int(i == j) for j in range(n)] for i in range(n)]
def get_matrix(n):
    return [[int(i == j) for j in range(n)] for i in range(n)]