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.

链表的中间节点。

题目即是题意，注意如果有两个中间节点（总节点个数为偶数）的话，返回第二个中间节点。

思路是快慢指针，快指针停下的时候，慢指针所在的位置即是所求。

时间O(n)

空间O(1)

Java实现

 1 class Solution {
 2     public ListNode middleNode(ListNode head) {
 3         ListNode slow = head;
 4         ListNode fast = head;
 5         while (fast != null && fast.next != null) {
 6             slow = slow.next;
 7             fast = fast.next.next;
 8         }
 9         return slow;
10     }
11 }

 

JavaScript实现

 1 /**
 2  * @param {ListNode} head
 3  * @return {ListNode}
 4  */
 5 var middleNode = function(head) {
 6     let slow = head;
 7     let fast = head;
 8     while (fast !== null && fast.next !== null) {
 9         slow = slow.next;
10         fast = fast.next.next;
11     }
12     return slow;
13 };

 

LeetCode 题目总结