0141.md

• 使用两个指针，快指针一次移动两步，慢指针一次移动一步，若链表有环，则二者必将相遇

class Solution {
 public:
  bool hasCycle(ListNode* head) {
    ListNode* slow = head;
    ListNode* fast = head;
    while (fast && fast->next) {
      slow = slow->next;
      fast = fast->next->next;
      if (slow == fast) {
        return true;
      }
    }
    return false;
  }
};