Linked List Cycle II -- Leetcode

Question:

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?

Answer:

class Solution {

public:

    ListNode *detectCycle(ListNode *head) {

        ListNode *slow=head, *fast=head;

    

       while(slow&&fast&&fast->next){

            slow=slow->next;

            fast=fast->next->next;

            if(slow==fast)

                break;

        }

        

        if(!slow||!fast||!fast->next){

            return NULL;

        }

        

        slow=head;

        while(slow!=fast){

            slow=slow->next;

            fast=fast->next;

        }

        return slow;

    }

};