Reverse a singly-linked list iteratively.
Examples
L = null, return null
L = 1 -> null, return 1 -> null
L = 1 -> 2 -> 3 -> null, return 3 -> 2 -> 1 -> null
class Solution(object):
def reverse(self, head):
if not head:
return None
p = head
q = head.next
while q:
head.next = q.next
q.next = p
p = q
q = head.next
return p
