LeetCode: Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Follow up:
Can you solve it without using extra space?

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 
     typedef ListNode ln;


     ListNode *detectCycle(ListNode *head) {  
     if(!head || !head->next)return NULL;  
     ln *fast = head, *slow = head;  
     while(fast && fast->next)  
     {  
       slow = slow->next;  
       fast = fast->next->next;  
       if(fast == slow)break;  
     }  
     if(fast != slow)return NULL;  
     ln *first = head, *second = slow;  
     while(first != second)  
     {  
       first = first->next;  
       second = second->next;  
     }  
     return first;  
   }