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) {}

 * };

 */

class Solution {

public:

    ListNode *detectCycle(ListNode *head) 

    {

        if(head==NULL || head->next==NULL) return NULL;

        //check if cycle exist

        ListNode* p1=head;

        ListNode* p2=head;

        while(true)

        {

            p1=p1->next;

            p2=p2->next;

            if(p2==NULL || p2->next==NULL) return NULL;

            p2=p2->next;

            if(p1==p2) break;

        }

        p2=head;

        while(p1!=p2)

        {

            p1=p1->next;

            p2=p2->next;

        }

        return p1;

    }

};