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?

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
         /*
        1. 空链表不成环
        2. 一个节点自环
        3. 一条链表完整成环
        */
        if(head==NULL)return false;
        if(head->next==NULL)return false;
        if(head->next==head)return true;

        ListNode* y=head->next;
        ListNode* x=head->next->next;
        while(x!=NULL && y!=NULL)
        {
            x=x->next;
            if(x==NULL)break;
            x=x->next;
            y=y->next;
            if(x==y)break;
        }
        return x==y;
    }
    
};