[LeetCode] Linked List Cycle

Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?

解题思路：
给两个指针，一个每次走一步，另外一个每次走2步。如果两者相遇，则有圈，否则没有圈

Java code

public boolean hasCycle(ListNode head) {
        ListNode p = head;
        ListNode pn = head;
        
        while(p != null && pn != null && pn.next != null){
             p = p.next;
             pn = pn.next.next;
             
             if(p != null && p == pn) return true;
        }
        
        return false;
    }