An integer N (1 ≤ N ≤ 2000), representing some quantity or size.
A list of integers A of size 3N, where each integer is between 1 and N.

### Example Input:

```
2
1 2 1 2 2 1
```

### Function Signature:
Write a function f(N, A) that takes in the input.
```python
def f(N: int, A: List[int]):
    '''
    N: an integer
    A: a list of integers
    '''
```