问题描述

Given the head of a singly linked list, return the middle node of the linked list.

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

测试样例

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Input: head = [1,2,3,4,5]
Output: [3,4,5]
Explanation: The middle node of the list is node 3.

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

说明

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The number of nodes in the list is in the range [1, 100].
1 <= Node.val <= 100

解题

思路

使用快慢指针，快指针到达链表尾时，慢指针正好为链表中央。

补充：

  1. 链表
  2. 时间复杂度 O(n)

代码

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode middleNode(ListNode head) {
        // 快慢指针
        ListNode slow = head, fast = head;
        
        while(fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        
        return slow;
    }
}