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
    '''
```