ListNode *detectCycle(ListNode *head) {

         if( head == NULL || head->next == NULL ){

             return NULL ;

         }

         ListNode* fast = head;

         ListNode* slow = head;

         while( fast != NULL && slow != NULL ){

             if( fast->next !=  NULL ){

                 fast = fast->next->next;

             }else{

                 return NULL ;

             }

             slow = slow->next;

             if( fast == slow ){

                 break;

             }

         }

         if( fast == NULL || slow == NULL ){

             return NULL;

         }

         ListNode* start = head;

         while( start != slow ){

             start = start->next ;

             slow = slow->next;

         }

         return start;

     }