给定一个链表，判断链表中是否有环。

如果连表中有某个节点，可以通过连续追踪next指针再次到达该节点，则链表中存在环

如果链表中存在环，则返回true，否则返回false

public class LinkCycle {
    static class ListNode{
        int val;
        ListNode next;

        public ListNode(int val, ListNode next) {
            this.val = val;
            this.next = next;
        }
    }
    public static void main(String[] args) {
        ListNode node5 = new ListNode(5,null);
        ListNode node4 = new ListNode(4,node5);
        ListNode node3 = new ListNode(3,node4);
        ListNode node2 = new ListNode(2,node3);
        ListNode node1 = new ListNode(1,node2);
//        node5.next = node3;
        System.out.println(hasCycle(node1));
    }
    public static boolean hasCycle(ListNode head){
        Set<ListNode> set = new HashSet<>();
        while(head != null){
            if (!set.add(head)){
                return true;
            }
            head = head.next;
        }
        return false;
    }
    //双指针
    public static boolean hasCycle1(ListNode head){
        if (head == null || head.next == null){
            return false;
        }
        ListNode slow = head;
        ListNode quick = head.next;
        while(slow != quick){
            if (quick == null || quick.next == null){
                return false;
            }
            slow = slow.next;
            quick = quick.next.next;
        }
        return true;
    }
}