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/**
 * 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) {
        return findOne(head);
    }
    ListNode *findOne(ListNode* head) {
        if (head == NULL || head->next == NULL) return NULL;
        ListNode *slow = head, *fast = head, *entry = head;
        while(slow && fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast) {
                slow = head;
                while(slow != fast) {
                    slow = slow->next;
                    fast = fast->next;
                }
                return fast;
            }
        }
        return NULL;
    }
};