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?

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
 };
 
class Solution {
public:
	 ListNode *detectCycle(ListNode *head) {
        bool flag = false;
        ListNode *fast, *slow;
    	fast = slow = head;
    	while(fast && fast->next) {
    		fast = fast->next->next;
    		slow = slow->next;
    		if(fast == slow) {
    			flag = true;
    			break;
    		}	
    	}
    	if(!flag) return NULL;
    	int count = 1;			// count表示环的长度 
    	fast = fast->next->next;
    	slow = slow->next;
    	//当fast与slow重合后，还需count-1次才能重新重合 
    	while(fast != slow) {
    		fast = fast->next->next;
    		slow = slow->next;
    		count++;	
    	}
    	fast = slow = head;
    	while(count--) 	fast = fast->next;	
		while(fast != slow) {
			fast = fast->next;
			slow = slow->next;
		}
		return fast;	   
    }
};