142. 环形链表 II

class Solution 
{
public:
    ListNode *detectCycle(ListNode *head) 
    {
        ListNode *slow = head, *fast = head;
        while (fast != nullptr) 
        {
            slow = slow->next;
            if (fast->next == nullptr) 
            {
                return nullptr;
            }
            fast = fast->next->next;
            if (fast == slow) 
            {
                ListNode *ptr = head;
                while (ptr != slow) 
                {
                    ptr = ptr->next;
                    slow = slow->next;
                }
                return ptr;
            }
        }
        return nullptr;
    }
};