206 Reverse Linked List

Original Description: Reverse a singly linked list.

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• 要求：
  reverse 一个单向链表

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• 返回： reverse后的链表

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• 例子：
  • input: 1->2->3->4->5->NULL
  • output: 5->4->3->2->1->NULL

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Solution

设置prev,每走一步，将指向next改为指向prev.

# 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
        """
        
        if head == None:
            return []
        else:
            prev = None
            curr = head
            after = head.next

            while after != None:
                after = curr.next
                curr.next = prev
                prev = curr
                curr = after
                after = after.next

            curr.next = prev 
            return curr