杭电1269----迷宫城堡

//tarjan裸题
#include <iostream>
#include <vector>
#include <stack>
#include <cstring>
using namespace std;
const int maxn = 1e4+5;
stack<int> s;
vector<int> v[maxn];
int dfn[maxn],low[maxn],ins[maxn];
int ant,cnt;
int tarjan(int n)
{
    dfn[n] = low[n] = ++ant;
    s.push(n);
    for(int i = 0; i < v[n].size(); ++i)
    {
        int x = v[n][i];
        if(!dfn[x])
            low[n] = min(low[n],tarjan(x));
        else if(!ins[x])
            low[n] = min(low[n],dfn[x]);
    }
    if(low[n] == dfn[n])
    {
        ++cnt;
        int x;
        do
        {
            x = s.top(); s.pop();
            ins[x] = cnt;
        }while(x != n);
    }
    return low[n];
}
int main()
{
    int n,m,i;
    while(cin >> n >> m && n+m)
    {
        ant = cnt = 0;
        memset(dfn,0,sizeof dfn);
        memset(low,0,sizeof low);
        memset(ins,0,sizeof ins);
        for(i = 0; i <= n; ++i)
            v[i].clear();
        while(m--)
        {
            int x,y;
            cin >> x >> y;
            v[x].push_back(y);
        }
        tarjan(1);
        for(i = 1; i < n; ++i)
            if(ins[i] != ins[i+1])
                break;
        if(i == n)
            cout << "Yes" << endl;
        else
            cout << "No" << endl;
    }
    return 0;
}