3 - Easy - 反转链表

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)