struct ListNode *detectCycle(struct ListNode *head) {
    struct ListNode* fast = head;
    struct ListNode* slow = head;
    while(fast && fast->next)
    {
        slow = slow->next;
        fast = fast->next->next;
        if(fast == slow)
        {
            struct ListNode* meetNode = fast;
            struct ListNode* list = meetNode->next; 
            int lenA = 1;
            struct ListNode* head1 = head;
            struct ListNode* head2 = list;
            while(head1 != meetNode)
            {
                head1 = head1->next;
                lenA++;
            }
            int lenB = 1;
            while(head2 != meetNode)
            {
                head2 = head2->next;
                lenB++;
            }
            int gap = abs(lenA-lenB);
            struct ListNode* longList = head;
            struct ListNode* shortList = list;
            if(lenA < lenB)
            {
                longList = list;
                shortList = head;
            }
            while(gap--)
            {
                longList = longList->next;
            }
            while(longList != shortList)
            {
                longList = longList->next;
                shortList = shortList->next;
            }
            return longList;
    
        }
        
    }
    return NULL;
    
}