leetcode讲解--876. Middle of the Linked List

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题目

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.

题目地址

讲解

一看到这个题目我就知道用快慢指针。但具体操作的时候有些细节需要注意,要让快慢指针指向第0个位置,而不是head(head算作第一个位置)。

Java代码

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode middleNode(ListNode head) {
        System.out.println(head.val);
        ListNode slow = new ListNode(0);
        ListNode fast = new ListNode(0);
        slow.next = head;
        fast.next = head;
        while(slow!=null && fast!=null){
            slow = slow.next;
            if(fast.next!=null){
                fast = fast.next.next;
            }else{
                fast = fast.next;
            }
        }
        return slow;
    }
}

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