NC4 判断链表中是否有环（map容器解法）

class Solution {
public:
    bool hasCycle(ListNode *head) 
    {
        map<ListNode*, int> mp;
        while(head != NULL)
        {
            if(0 == mp[head])
            {
                mp[head] = 1;
            }
            else
            {
                return true;
            }
            head = head->next;
        }
        return false;
        
    }
};