/**
knakul853
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {

       ListNode* slow = head;
        ListNode* fast = head;
        bool has = false;

        while(fast && fast->next )
        {
            fast = fast->next->next;
             slow = slow->next;

            if(slow == fast){  has = true; break;}
        }
        if(!has) return NULL;
        slow = head;
        while(fast != slow )
        {
            fast = fast->next;
            slow = slow->next;
        }


        return slow;

    }
};