【图】拓扑排序

#include<iostream>
#include<vector>
#include<stack>
using namespace std;

class Hdu3342{
public:
	void initial(int n, int m);
	void read_case();
	bool topological_sort();
	void print_result();
private:
	vector<int> vec[101];//节点的邻接表 
	int in[101];//保存节点的入度 
	int member_num;
	int test_num;
};
void Hdu3342::initial(int n, int m){
	for (int i = 0; i < 101; i++){
		vec[i].clear();
	}
	memset(in, 0, sizeof(in));
	member_num = n;
	test_num = m;
}
void Hdu3342::read_case(){
	int m, n;
	for (int i = 0; i < test_num; i++){
		cin >> m >> n;
		vec[m].push_back(n);
		in[n]++;//入度加1 
	}
}
bool Hdu3342::topological_sort(){
	stack<int> stk;
	int count = 0;//统计输出顶点的个数 
	for (int i = 0; i < member_num; i++){
		if (in[i] == 0){
			stk.push(i);//将入度为0的顶点入栈 
		}
	}
	while (!stk.empty()){
		int tmp = stk.top();
		stk.pop();
		count++;
		for (int i = 0; i < vec[tmp].size(); i++){//对此顶点弧表遍历 
			if (--in[vec[tmp][i]] == 0)
				stk.push(vec[tmp][i]);
		}
	}
	if (count == member_num)//如果count小于member_num,说明存在环 
		return true;
	return false;
}
void Hdu3342::print_result(){
	if (topological_sort())
		cout << "YES" << endl;
	else
		cout << "NO" << endl;
}
int main(){
	Hdu3342 x;
	int m, n;
	while (cin >> m >> n && (m || n)){
		x.initial(m, n);
		x.read_case();
		x.print_result();
	}
	return 0;
}