牛客网：BM6 判断链表中是否有环

import java.util.HashSet;
import java.util.Map;



public class Solution {
    public static void main(String[] args) {
        Solution solution = new Solution();

    }

    
    public boolean hasCycle(ListNode head) {
        HashSet map = new HashSet<>();
        while (head != null){
            if (map.contains(head)){
                return true;
            }
            map.add(head);
            head = head.next;
        }
        return false;
    }

    
    public boolean hasCycle2(ListNode head) {
        if (head == null){
            return false;
        }

        ListNode slow = head;
        ListNode last = head;
        while (last != null && last.next != null){
            last = last.next.next;
            slow = slow.next;
            if (slow == last){
                return true;
            }
        }
        return false;
    }
}

  class ListNode {
      int val;
      ListNode next;
      ListNode(int x) {
          val = x;
          next = null;
      }
  }