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?

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/** 

 * Definition for singly-linked list. 

 * class ListNode { 

 *     int val; 

 *     ListNode next; 

 *     ListNode(int x) { 

 *         val = x; 

 *         next = null; 

 *     } 

 * } 

 */  

public class Solution {  

    public boolean hasCycle(ListNode head) {  

        if(head==null) return false;          

        ListNode slow = head.next;  

        ListNode fast = null;  

        if(slow==null) return false;  

        else fast = slow.next;  

          

        while(slow!=null&&fast!=null){  

            if(slow==fast) return true;  

            else{  

                slow = slow.next;  

                if(fast.next!=null) fast = fast.next.next;  

                else return false;  

            }  

        }  

        return false;  

    }  

} 

原文链接http://blog.csdn.net/crazy__chen/article/details/46563843