142题

方法一：哈希表—将指针指向的节点，去哈希表中查找，若没有将节点放入哈希表中

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        ListNode *slow=head;
        ListNode *result=NULL;
        unordered_set<ListNode *>s;
        s.emplace(head);
        while(slow){
            slow=slow->next;
            if(s.count(slow)){
                result=slow;
                break;
            }
            s.emplace(slow);
        }
        return result;
    }
};