206 reverse linked list
206. Reverse Linked List
题目: https://leetcode.com/problems/reverse-linked-list/
难度: Easy
用三个指针，分别指向prev，cur 和 nxt，然后loop一圈还算比较简单.
class Solution(object):
def reverseList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
prev = None
cur = head
while(cur):
nxt = cur.next
cur.next = prev
prev = cur
cur = nxt
return prev
其实一个指针就够了
class Solution(object):
def reverseList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if not head:
return head
prev = None
while head.next:
tmp = head.next
head.next = prev
prev = head
head = tmp
head.next = prev
return head
递归版本，可以再消化一下.
class Solution(object):
def reverseList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
return self.reverseHelper(head, None)
def reverseHelper(self, head, new_head):
if not head:
return new_head
nxt = head.next
head.next = new_head
return self.reverseHelper(nxt, head)
