环形链表

 1 bool hasCycle(ListNode *head) {
 2     unordered_map<ListNode*, bool> map;
 3     ListNode* temp = head;
 4     bool res = false;
 5     while(temp!=nullptr)
 6     {
 7         if (map.count(temp))
 8         {
 9             res = true;
10             break;
11         }
12         map[temp] = true;
13         temp = temp->next;
14     }
15     return res;
16 }

 1 bool hasCycle(ListNode *head) {
 2     ListNode* fast=head;
 3     ListNode* slow = head;
 4     bool res = false;
 5     while (fast&&fast->next)
 6     {
 7         fast = fast->next->next;
 8         slow = slow->next;
 9         if (fast == slow)
10         {
11             res = true;
12             break;
13         }
14     }
15     return res;
16 }