/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */


class Solution {
public:
    /*
     * @param head: The first node of linked list.
     * @return: True if it has a cycle, or false
     */
    bool hasCycle(ListNode * head) {
        // write your code here
        ListNode *fast=head,*slow=head;
        while(fast&&fast->next)
        {
            fast=fast->next->next;
            slow=slow->next;
            if(slow==fast)
            return true;
        }
        return false;
    }
};