text: /**
   * 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) {
      ListNode *slow = head;
      ListNode *fast = head;
      while (fast && fast->next) {
        fast = fast->next->next;
        slow = slow->next;
        if (fast == slow) {
          ListNode *pos = head;
          while (pos != slow) {
            pos = pos->next;
            slow = slow->next;
          }
          return pos;
        }
      }
      return nullptr;
    }
  };
  