Description

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.

Solutions

  找到链表的中间值。

1. Two Pointers

  使用快慢指针即可。

# Time: O(n)
# Space: O(1)
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def middleNode(self, head: ListNode) -> ListNode:
        if not head or (head and not head.next):
            return head
        fast, slow = head, head
        while fast:
            if not fast.next:
                break
            fast = fast.next.next
            slow = slow.next
        return slow
# Runtime: 32 ms, faster than 35.49% of Python3
# Memory Usage: 13.6 MB, less than 7.14% of Python3

References

  1. 876. Middle of the Linked List