判断链表中是否有环

import java.util.;
/*

• 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) {

    List list = new ArrayList<ListNode>();
    while(head!=null){
        if(list.contains(head)){
            return true;
        }else{
            list.add(head);
            head=head.next;
        }
    }
    return false;

  }
  }