103. 带环链表 II.cpp

class Solution 
{
public:
    ListNode *detectCycle(ListNode *head) 
    {
        ListNode *fast, *slow ;  
        slow = fast = head ;  
        while (fast != NULL&&fast->next!=NULL)  
        {  
            slow = slow -> next ;  
            fast = fast -> next -> next ;  
            if (slow == fast) 
                break ;  
        }  
        if (fast == NULL||fast->next==NULL) 
            return NULL ; 
        ListNode * ptr1 = head ; 
        ListNode * ptr2 = slow ; 
        while (ptr1 != ptr2)   
        {  
            ptr1 = ptr1 -> next ;  
            ptr2 = ptr2 -> next ;  
        }  
        return ptr1 ; 
    }
};