142. Linked List Cycle II

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode *detectCycle(ListNode *head) {
12         ListNode* fast=head;
13         ListNode* slow=head;
14         while(1){
15             if(fast==NULL||fast->next==NULL)
16                 return nullptr;
17             fast=fast->next->next;
18             slow=slow->next;
19             if(fast==slow)
20                 break;
21         }
22         ListNode* p=head;
23         while(p!=slow){
24             p=p->next;
25             slow=slow->next;
26         }
27         return slow;
28     }
29 };