题解 | #判断链表中是否有环#

//快慢指针要以快指针作为判断条件
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
  public:
    bool hasCycle(ListNode* head) {
        if (head == nullptr || head->next == nullptr) return false;
        ListNode* first;
        ListNode* second;
        first = head;
        second = head;
        while (second->next && second->next->next) {
            first = first->next;
            second = second->next->next;
            if (first == second)
                return true;
        }
        return false;
    }
};