查找链表中是否有环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?

不使用额外空间：这里没有实例化一个对象，只是引用对象，所以不会开辟新内存空间

用一个快指针和一个慢指针，当两个指针相遇，说明有环

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

        return false;
    }
}