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 }