Leetcode-876

Leecode-876 Middle of the Linked List

思路：快慢指针

题目描述

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

Input: [1,2,3,4,5]
Output: 3 (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.

Solution：快慢指针

Java

Solution :

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode middleNode(ListNode head) {
        if(head == null) return null;
        ListNode slow = head;
        ListNode fast = head;

        while(fast != null && fast.next!=null){
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
}

Python

Solution :

# 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 head is None:
            return None
        slow = head
        fast = head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        return slow

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