35. Reverse Linked List/206. Reverse Linked List

• 本題難度: Easy
• Topic: Linked List

Description

Reverse a linked list.

Example
Example1:
For linked list 1->2->3, the reversed linked list is 3->2->1
Example2:
For linked list 1->2->3->4, the reversed linked list is 4->3->2->1

Challenge
Reverse it in-place and in one-pass

我的代碼

# 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):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head is None:
            return None
        before = None
        pos = head
        while(pos.next):
            after = pos.next
            pos.next = before
            before = pos
            pos = after
        pos.next = before
        return pos
 

思路

重點是Reverse it in-place and in one-pass。
處理好pos，before，after的關系，理順
註意：

1. null

• 時間復雜度 O(n)

[Lintcode]35. Reverse Linked List/[Leetcode]206. Reverse Linked List