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

@TOC

# 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.

# 解题方法

# 使用哑结点

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

class Solution:
"""
:rtype: ListNode
"""
dummy = ListNode(0)
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

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

class Solution:
"""