LeetCode - 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         if (head == NULL || head->next == NULL)
13             return NULL;
14             
15         ListNode *slow = head->next;
16         ListNode *fast = head->next->next;
17         
18         while (fast && fast != slow)
19         {
20             slow = slow->next;
21             fast = fast->next ? fast->next->next : fast->next;
22         }
23         
24         if (fast == NULL)
25             return NULL;
26             
27         for (fast = head; fast != slow; fast = fast->next)
28             slow = slow->next;
29         
30         return slow;
31     }
32 };