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 }