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?

题解：

问题：判断链表是否有环。
分析：利用快慢指针slow,fast
         slow指针每次走一步，fast指针每次走两步，倘若存在环，则slow和fast必定在某一时刻相遇。
         由于fast指针走的比slow快所以循环的时候只需要判断fast和fast->next不为空，判断fast->next是因为防止出现fast->NULL->next这种情况

code：

/**
 * 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,fast;
	     slow = head;
	     fast = head.next;
	     while(fast!=null && fast.next!=null)
	     {
	    	 if(slow==fast) return true;
	    	 slow = slow.next;
	    	 fast = fast.next.next;
	     }
	      return false;
    }
}