876. Middle of the Linked List 链表的中间结点

作者： 负雪明烛 id： fuxuemingzhu 个人博客： http://fuxuemingzhu.cn/

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题目地址：https://leetcode.com/problems/middle-of-the-linked-list/description/

题目描述

Given a non-empty, singly linked list with head node head, return a middle node of linked list.

If there are two middle nodes, return the second middle node.

Example 1:

Input: [1,2,3,4,5]
Output: Node 3 from this list (Serialization: [3,4,5])
The returned node has value 3.  (The judge's serialization of this node is [3,4,5]).
Note that we returned a ListNode object ans, such that:
ans.val = 3, ans.next.val = 4, ans.next.next.val = 5, and ans.next.next.next = NULL.

Example 2:

Input: [1,2,3,4,5,6]
Output: Node 4 from this list (Serialization: [4,5,6])
Since the list has two middle nodes with values 3 and 4, we return the second one.

Note:

• The number of nodes in the given list will be between 1 and 100.

题目大意

给出链表中的中间节点。如果链表长度为偶数，返回的应该是中间的第二个节点。

解题方法

使用哑结点

这个题很简单，就是快慢指针两个移动即可。

需要注意的是，如果链表长度为偶数，返回的应该是中间的第二个节点。这个做的方法，是判断fast指针是指向了最末尾的None还是链表结尾的node。如果是链表结尾说明是偶数个点，如果是none说明是奇数个点。

代码如下：

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

class Solution:
    def middleNode(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        dummy = ListNode(0)
        dummy.next = head
        slow, fast = dummy, dummy
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        return slow.next if fast else slow

不使用哑结点

如果不使用哑结点，初始时直接将两个指针指向head能获得更快的速度。这么做的前提是题目已经告诉了节点的数量最少是1，否则如果初始化是空的话fast.next会报错。

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

class Solution:
    def middleNode(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        slow, fast = head, head
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
        return slow

日期

2018 年 8 月 16 日 —— 一个月不写题，竟然啥都不会了。。加油！ 2018 年 11 月 6 日 —— 腰酸背痛要废了