142. 环形链表

 1 class Solution {
 2 public:
 3     ListNode *detectCycle(ListNode *head) {
 4         set<ListNode *> node_set;
 5         while (head) {
 6             if (node_set.find(head) != node_set.end()) {
 7                 return head;
 8             }
 9             node_set.insert(head);
10             head = head->next;
11         }
12         return NULL;
13     }
14 };

 1 class Solution {
 2 public:
 3     ListNode *detectCycle(ListNode *head) {
 4         ListNode *fast = head;
 5         ListNode *slow = head;
 6         ListNode *meet = NULL;//相遇的节点
 7         
 8         while (fast) {
 9             slow = slow->next;//各走一步
10             fast = fast->next;
11             if (!fast) {//fast到了尾部，说明不是环形
12                 return NULL;
13             }
14             fast = fast->next;
15             if (fast == slow) {
16                 meet = fast;
17                 break;
18             }
19         }
20         if (meet == NULL) {
21             return NULL;
22         }
23         while (head && meet) {//能到这里说明一定有环，条件(true)
24             if (head == meet) {
25                 return head;//相遇即环的起点
26             }
27             head = head->next;
28             meet = meet->next;
29         }
30         return NULL;//
31     }
32 };