快慢指针

思路

• 如果链表中有环，则快慢指针必定会相遇，每次迭代，fast执行两次next，slow就执行一次next；

• 注意：Java在While循环中，判断fast执行两次后next是否为null时，需要先判断执行一次next是否非null

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