hdu 1269 迷宫城堡 强连通分量

#include<cstdio>
#include<iostream>
#include<sstream>
#include<cstdlib>
#include<cmath>
#include<cctype>
#include<string>
#include<cstring>
#include<algorithm>
#include<stack>
#include<queue>
#include<set>
#include<map>
#include<ctime>
#include<vector>
#include<fstream>
#include<list>
using namespace std;

#define ms(s) memset(s,0,sizeof(s))
typedef unsigned long long ULL;
typedef long long LL;

const double PI = 3.141592653589;
const int INF = 0x3fffffff;

int n, m;

const int maxn = 10010;
stack<int> stk;
vector<int> G[maxn];
int dfn[maxn], low[maxn], vis[maxn];
int times, tot;//强连通分量的个数

void tarjan(int u){
dfn[u] = low[u] = ++times;
stk.push(u);  vis[u] = -1;
for(int i = 0; i < G[u].size(); ++i){
int v = G[u][i];
if(vis[v] == 0){
tarjan(v);
low[u] = min(low[u], low[v]);
}
else{
if(vis[v] == -1)
low[u] = min(low[u], dfn[v]);
}
}
if(dfn[u] == low[u]){//u是一个强连通分量的根
tot++;
while(!stk.empty()){
int v = stk.top();  stk.pop();
vis[v] = 1;
if(v == u)
break;
}
}
}
void solve(int n){
for(int i = 1; i <= n; ++i){
if(vis[i] == 0)
tarjan(i);
}
}

int main(){
//    freopen("E:\\input.txt","r",stdin);
//    freopen("/home/really/Document/output","w",stdout);
//    ios::sync_with_stdio(false);

int a,b;
while(scanf("%d%d",&n,&m) && (n||m)){
times = tot = 0;
ms(vis);
while(!stk.empty())
stk.pop();
for(int i = 1; i <= n; ++i)
G[i].clear();
for(int i = 0; i < m; ++i){
scanf("%d%d",&a,&b);
G[a].push_back(b);
}
solve(n);
if(tot <= 1)
printf("Yes\n");
else
printf("No\n");
}

return 0;
}