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<unordered_map>

class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode* CountPoint = head;
	  //定义一个哈希表
        unordered_map<ListNode*,int>mymap;
        if(head->next ==nullptr){return false;}
        if(head==nullptr){return false;}
        CountPoint = CountPoint->next;
       //判断mymap是否出现重复元素,先走一遍链表，结束条件为出现重复元素或者最后一个元素指向Null
        while(mymap[CountPoint]<1 && CountPoint->next !=nullptr){
            mymap[CountPoint]++;
            CountPoint = CountPoint->next;           
        }
        if (CountPoint->next==nullptr) {return false;
        }
        else {
         return true;
        }        
    }
};