I was getting a mysterious error at an input length of 20000 for my Python solution. It turns out to be because I'd implemented linked list length counting recursively and I was hitting a stack depth limit. It would be great if the error message were more clear, because this is a totally valid thing to write for an interview:

```
def length(node):
return 0 if not node else 1 + length(node.next)
```

Bah.