Linked List Cycle (Java)

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

设两个指针，一个走两步，一个走一步，如果有环，快的一定会和慢的汇合

Source

/** * 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 p = head, q = head;                while(q.next != null && q.next.next != null){  //由于此题不是找中间位置，所以可以写为判断本身和next的值是否为null，不影响        p = p.next;        q = q.next.next;        if(p == q) return true;        }               return false;    }}