142. Linked List Cycle II

I'm inspired by this blog which has very detailed explanation.

1:  class Solution {  
2:  public:  
3:    ListNode *detectCycle(ListNode *head) {  
4:      ListNode *slow = head, *fast = head;  
5:      while (slow && fast) {  
6:        slow = slow->next;  
7:        if (fast->next == NULL) return NULL;  
8:        fast = fast->next->next;  
9:        if (slow == fast) break;  
10:      }  
11:      if (fast == NULL) return NULL;  
12:      fast = head;  
13:      while (slow != fast) {  
14:        slow = slow->next;  
15:        fast = fast->next;  
16:      }  
17:      return slow;  
18:    }  
19:  };