题解 | #判断链表中是否有环#

/**
 * 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) {
        set<ListNode *> s;

        while(head)
        {
            if(!s.empty() && s.count(head)!=0)
                return true;
            s.emplace(head);
            head = head->next;
        }

        return false;
    }
};