Leetcode: Linked List Cycle

Given a linked list, determine if it has a cycle in it.



Follow up:

Can you solve it without using extra space?

Analysis: typical Runner Technique. 一次过

 1 /**

 2  * Definition for singly-linked list.

 3  * class ListNode {

 4  *     int val;

 5  *     ListNode next;

 6  *     ListNode(int x) {

 7  *         val = x;

 8  *         next = null;

 9  *     }

10  * }

11  */

12 public class Solution {

13     public boolean hasCycle(ListNode head) {

14         if (head == null) return false;

15         ListNode current = head;

16         ListNode runner = head;

17         while (runner != null && runner.next != null) {

18             runner = runner.next.next;

19             current = current.next;

20             if (runner == current) return true;

21         }

22         return false;

23     }

24 }