python_leetcode206. 反转链表_leetcode206反转链表python

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？

（一）迭代

# 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 not head:
            return None
        p = head
        q = head.next
        p.next = None
        while q:
            r = q.next
            q.next = p
            p = q
            q = r
        return p

（二）递归

    if head == None or head.next == None:

        return head

    root = self.reverseList(head.next)

    head.next.next = head

    head.next = None

    return root