public ListNode detectCycle(ListNode head) {

    ListNode fast = head;

    ListNode slow = head;

    int flag = 0;

    ListNode intersection = null;

    while(fast != null && fast.next != null){

        fast = fast.next.next;

        slow = slow.next;

        if(fast == slow){

            intersection = fast;

            flag = 1;

            break;

        }

    }

    if(flag == 0 ) return null;

    slow = head;

    while(slow != fast){

        slow = slow.next;

        fast = fast.next;

    }

    return fast;

    }