[leetcode] Linked 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) return false;
ListNode *p1=head, *p2=head->next;
while(p2) {
p1 = p1->next;
if(p2->next) {
p2=p2->next->next;
} else return false;
if(p1 == p2) return true;
}
return p1==p2;
}
};