LeetCode Reverse Linked List （反置链表）

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
def reverseList(self, head, pre=None):
"""
:type head: ListNode
:rtype: ListNode
"""
if not head:    return pre
back=head.next
head.next=pre
return self.reverseList(back, head)