Reversing a linked list in python

Question

I am asked to reverse a which takes head as parameter where as head is a linked list e.g.: 1 -> 2 -> 3 which was returned from a function already defined I tried to implemen

Answer 1

Here is a way to reverse the list 'in place'. This runs in constant time O(n) and uses zero additional space.

def reverse(head):
  if not head:
    return head
  h = head
  q = None
  p = h.next
  while (p):
    h.next = q
    q = h
    h = p
    p = h.next
  h.next = q
  return h

Here's an animation to show the algorithm running.
(# symbolizes Null/None for purposes of animation)