leetcode[142]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)return NULL;
        ListNode *slow=head;
        ListNode *fast=head;
        bool flag=false;
        while(fast&&fast->next)
        {
            fast=fast->next->next;
            slow=slow->next;
            if(fast==slow)
            {
                flag=true;
                break;
            }
        }
        if(flag)
        {
            fast=head;
            while(fast!=slow)
            {
                fast=fast->next;
                slow=slow->next;
            }
            return fast;
        }
        return NULL;
    }
};