206.py

"""
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？

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reverse-linked-list
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。
"""

# Definition for singly-linked list.
class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None


class Solution:

    # 递归法
    def reverseList(self, head: ListNode) -> ListNode:
        if head is None or head.next is None:
            return head
        new_head = self.reverseList(head.next)
        head.next.next = head
        head.next = None
        return new_head

    # 迭代法
    def reverseList(self, head: ListNode) -> ListNode:
        prev_node, curr_node = None, head
        while curr_node:
            next_node = curr_node.next
            curr_node.next = prev_node
            prev_node = curr_node
            curr_node = next_node

        return prev_node