leetcode141 Linked List Cycle

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode
 4  * {
 5  *     int val;
 6  *     ListNode *next;
 7  *     ListNode(int x) : val(x), next(NULL) {}
 8  * };
 9  */
10 class Solution 
11 {
12 public:
13     bool hasCycle(ListNode *head)
14     {
15         if (!head) return false;
16         ListNode *p = head, *q = head;
17         while (p && q)
18         {
19             if (!p->next || !p->next->next) break;
20             p = p->next->next;
21             q = q->next;
22             if (p == q) return true;
23         }
24         return false;
25     }
26 }