206. 反转链表--PYTHON

宋建国

于 2019-12-25 18:36:38 发布

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分类专栏： LEETCODE

本文链接：https://blog.youkuaiyun.com/hot7732788/article/details/103678455

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本文探讨了链表逆序的两种实现方法：暴力破解法和链表递归重组法。暴力破解法将链表元素转为数组再逆序重组；链表递归重组法则直接在链表上进行逆序操作，更为高效。

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自己1：暴力破解法

# 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
        
        num_list = list()
        p = head
        while p:
            num_list.append(int(p.val))
            p = p.next
        num_list = num_list[::-1]

        head_ = ListNode(-1)
        p = head_
        for i in num_list:
            tmp = ListNode(i)
            p.next = tmp
            p = p.next
        return head_.next

自己2：链表递归重组法

# 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

        res = ListNode(-1)
        res.next = self.merg_(head)
        return res.next    
    
    def merg_(self,head):
        if not head.next:
            return head

        head_ = self.merg_(head.next)
        tail = head
        tail.next = None

        p = head_
        while p.next:
            p = p.next
        p.next = tail
        return head_