Q： Reverse a singly linked list. Hint: A linked list can be reversed either iteratively or recursively. Could you implement both?
A：
class ListNode(object):
def __init__(self, x):
self.val = x
self.next =
None
class Solution(object):
def reverseList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
pre =
None
while head:
current = head
head = head.next
current.next = pre
pre = current
return pre
