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