// 思路：快慢指针 + 回拨指针到头节点
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if (head == NULL || head->next == NULL) {
            return NULL;
        }

        auto slow = head;
        auto fast = head;
        while(slow && fast) {
            slow = slow->next;
            fast = fast->next;
            if (fast) 
                fast = fast->next;

            if (slow == fast)
                break;
        }

        if (fast == nullptr) {
            return nullptr;
        }

        auto headA = slow;
        while(headA != head) {
            headA = headA->next;
            head = head->next;
        }

        return headA;
    }
};