Question

I have been working on a recursively defined list class in Python and I’m having trouble coding a reverse() method to function recursively. Here’s the basis for the class.

class RecList:
  def __init__(self):
    self._head = None
    self._rest = None

The base case would be self._head, being the first entry in a list, followed by the recursive case which is essentially another list containing its own self._head to start, then defined recursively. This proceeds all the way down to the bottom level where both self._head and self._rest = None. Is there a simple way to code a reverse method for a list defined like this?

Answer 1

Try this:

class RecList:
  def __init__(self, head=None, rest=None):
    self._head = head
    self._rest = rest
  def __str__(self):
    if self._rest is None:
        return str(self._head)
    return str(self._head) + ' ' + self._rest.__str__()
  def reverse(self):
    return self._reverse_aux(None)
  def _reverse_aux(self, acc):
    if self._rest is None:
        return RecList(self._head, acc)
    return self._rest._reverse_aux(RecList(self._head, acc))

lst = RecList(1, RecList(2, RecList(3, None)))
print lst
> 1 2 3
print lst.reverse()
> 3 2 1