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) {}
 * };
 */
#include <map>
class Solution {
public:
    bool hasCycle(ListNode *head) {
        std::map<ListNode*, bool> mem;
        for(auto cur = head; cur; cur = cur->next) {
            if(mem.count(cur)) return true;
            mem.emplace(cur, true);
        }
        return false;
    }
};