算法思路（主要是数学推导）：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        int count = 0;
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast != nullptr && fast->next != nullptr)
        {
            slow = slow->next;
            fast = fast->next->next;

            if(slow == fast) 
            {
                ListNode* ptr = head;//快慢指针相遇后，在链表头节点创建新指针，它和慢指针一起跑一定会第一次就在环的开始相遇
                while(slow != nullptr && slow->next != nullptr)
                {
                    if(slow == ptr) return ptr;
                    slow = slow->next;
                    ptr = ptr->next;
                }
            }
        
        }
        return nullptr;
    }
};