[HDU] 迷宫城堡 HDU - 1269

 1 #include<cstdio>
 2 #include<iostream>
 3 #include<cstring>
 4 #define maxn 1000000
 5 using namespace std;
 6 
 7 struct edge{
 8     int from,v;
 9 }e[maxn];
10 
11 int tot,first[maxn];
12 void insert(int u,int v){
13     tot++;
14     e[tot].from = first[u];
15     e[tot].v = v;
16     first[u] = tot;
17 }
18 
19 bool vis[maxn];
20 int color[maxn],n,m,dfn[maxn],low[maxn],TIM,COL,top,stack[maxn];
21 
22 void Tarjan(int p){
23     dfn[p] = low[p] = ++TIM;
24     vis[p] = true;
25     stack[++top] = p;
26     for(int i = first[p];i;i = e[i].from){
27         int v = e[i].v;
28         if(!dfn[v]){
29             Tarjan(v);
30             low[p] = min(low[p],low[v]);
31         }else if(vis[v]) low[p] = min(low[p],dfn[v]);
32     }
33     
34     if(dfn[p] == low[p]){
35         ++COL;
36         color[p] = COL;
37         vis[p] = false;
38         while(stack[top] != p){
39             color[stack[top]] = COL;
40             vis[stack[top--]] = false;
41         }
42         top--;
43     }
44     
45 //    printf("dfn[%d]: %d low[%d]: %d\n",p,dfn[p],p,low[p]);
46     
47 }
48 
49 int main(){
50     
51     while(1){
52         memset(color,0,sizeof(color));
53         memset(dfn,0,sizeof(dfn));
54         memset(low,0,sizeof(low));
55         memset(first,0,sizeof(first));
56         tot = COL = TIM = top = 0;
57         scanf("%d%d",&n,&m);
58         
59         if(!n && !m) return 0;
60         
61         int a,b;
62         for(int i = 1;i <= m;i++){
63             scanf("%d%d",&a,&b);
64             insert(a,b);
65         }
66         
67         for(int i = 1;i <= n;i++) if(!dfn[i]) Tarjan(i);
68         
69         if(COL == 1) printf("Yes\n",COL);
70         else printf("No\n");
71     }
72     
73     
74     return 0;
75 }