leetcode-141-link list cycle

/**
 * 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) {
        if(head == NULL || head->next == NULL)
            return false;
        
        ListNode* p_ahead = head;
        ListNode* p_behind = head;
        
        while(p_ahead && p_ahead->next){
            p_ahead = p_ahead->next->next;
            p_behind = p_behind->next;
            if(p_ahead == p_behind){
                return true;
            }
        }
        
        return false;
    }
};

千万不要再while判断里面加上p->next->next这种连续两次next的判断，一次只想对判断next一次