[leedcode 141] 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?

/**
* 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) {
//双指针，一快一慢，绝对速度为1，如果相遇 有环，马上返回，fast==null或者fast为尾节点时，返回false
if(head==null) return false;
ListNode fast=head;
ListNode slow=head;
while(fast!=null&&fast.next!=null){
fast=fast.next.next;
slow=slow.next;
if(fast==slow) return true;
}
return false;
}
}