HDU 1269 迷宫城堡

#include <iostream>
#include <algorithm>
#include <cstring>
#include <cstdio>
#include <vector>
#include <queue>
#include <stack>
#include <cstdlib>
#include <iomanip>
#include <cmath>
#include <cassert>
#include <ctime>
#include <map>
#include <set>
using namespace std;
#pragma comment(linker, "/stck:1024000000,1024000000")
#define lowbit(x) (x&(-x))
#define max(x,y) (x>=y?x:y)
#define min(x,y) (x<=y?x:y)
#define MAX 100000000000000000
#define MOD 1000000007
#define pi acos(-1.0)
#define ei exp(1)
#define PI 3.1415926535897932384626433832
#define ios() ios::sync_with_stdio(true)
#define INF 0x3f3f3f3f
#define mem(a) ((a,0,sizeof(a)))
typedef long long ll;
int n,m,vis[10005],dnf[10005],low[10005],pos[10006];
int k,top,x,y;
vector<int>v[10005];
void tarjan(int u)
{
    low[u]=dnf[u]=++k;
    pos[++top]=u;
    vis[u]=1;
    for(int i=0;i<v[u].size();i++)
    {
        if(!dnf[v[u][i]])
        {
            tarjan(v[u][i]);
            low[u]=min(low[u],low[v[u][i]]);
        }
        else if(vis[v[u][i]]) low[u]=min(low[u],dnf[v[u][i]]);
    }
}
int main()
{
    while(scanf("%d%d",&n,&m) && n+m)
    {
        memset(vis,0,sizeof(0));
        memset(dnf,0,sizeof(dnf));
        memset(low,0,sizeof(low));
        top=k=0;
        for(int i=0;i<=n;i++)
            v[i].clear();
        for(int i=0;i<m;i++)
        {
            scanf("%d%d",&x,&y);
            v[x].push_back(y);
        }
        tarjan(1);
        bool flag=true;
        for(int i=2;i<=n;i++)
            if(low[i]!=low[i-1]){flag=false;break;}
        puts(flag?"Yes":"No");
    }
    return 0;
}