[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?

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