List Cycle

 https://www.interviewbit.com/problems/list-cycle/

ListNode* Solution::detectCycle(ListNode* A) {

    //Floyd's cycle finding algorithm

    if(!A  || !A->next){

        return NULL;

    }

    ListNode* slow;

    ListNode* fast;

    slow=fast=A;

    while(fast){

        if(!fast->next || !fast->next->next){

            return NULL;

        }

        fast=fast->next->next;

        slow=slow->next;

        if(fast==slow) break;

    }

    slow=A;

    while(slow!=fast){

        slow=slow->next;

        fast=fast->next;

    }

    return slow;

}