反转一个单链表。
示例:
输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL
进阶:
你可以迭代或递归地反转链表。你能否用两种方法解决这道题？
循环
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution:
def reverseList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
node = head
pre = None
while node:
_next = node.next
node.next = pre
pre = node
node = _next
return pre
递归
class Solution:
def recursive(self, pre, node):
if node == None:
return pre
else:
_next = node.next
node.next = pre
return self.recursive(node, _next)
def reverseList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
return self.recursive(None, head)
