Linked List Cycle

一次AC

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) 
    {
        ListNode *p, *q;
        if(!head||!head->next)
            return false;
        p = head;
        q = head->next;
        while(q)
        {
            if(p->val!=q->val)
            {
                p = p->next;
                if(!q->next)
                    return false;
                q = q->next->next;
            }
            else
             return true;
        }
        return false;
    }
};