传送门：http://acm.hdu.edu.cn/showproblem.php?pid=1269
解题思路：
如果整个图只有一个连通分量输出“Yes”，其它就输出“NO”
1 #include <iostream> 2 #include <cstdio> 3 #include <cstring> 4 #include <cstdlib> 5 #include <vector> 6 #include <algorithm> 7 using namespace std; 8 9 const int MAXN=100010; 10 const int MAXM=2000010; 11 12 struct Edge{ 13 int to,next; 14 }edges[MAXM]; 15 int head[MAXN],tot; 16 17 void init(){ 18 memset(head,-1,sizeof(head)); 19 tot=0; 20 } 21 22 void addEdge(int u,int v){ 23 edges[tot].to=v; 24 edges[tot].next=head[u]; 25 head[u]=tot++; 26 } 27 28 int DFN[MAXN],LOW[MAXN],Belong[MAXN],Stack[MAXN]; 29 int Index,top; 30 int scc; 31 bool Instack[MAXN]; 32 int num[MAXN]; 33 34 void Tarjan(int u){ 35 int v; 36 LOW[u]=DFN[u]=++Index; 37 Stack[top++]=u; 38 Instack[u]=true; 39 40 for(int i=head[u];i!=-1;i=edges[i].next){ 41 v=edges[i].to; 42 43 if(!DFN[v]){ 44 Tarjan(v); 45 if(LOW[u]>LOW[v]) LOW[u]=LOW[v]; 46 }else if(Instack[v]&&LOW[u]>DFN[v]) 47 LOW[u]=DFN[v]; 48 } 49 50 if(LOW[u]==DFN[u]){ 51 scc++; 52 53 do{ 54 v=Stack[--top]; 55 Instack[v]=false; 56 num[scc]++; 57 Belong[v]=scc; 58 }while(v!=u); 59 } 60 } 61 62 void solve(int N){ 63 memset(DFN,0,sizeof(DFN)); 64 memset(Instack,false,sizeof(Instack)); 65 memset(num,0,sizeof(num)); 66 67 Index=top=scc=0; 68 69 for(int i=1;i<=N;i++){ 70 if(!DFN[i]) 71 Tarjan(i); 72 } 73 74 if(scc>1) 75 cout<<"No"<<endl; 76 else 77 cout<<"Yes"<<endl; 78 } 79 80 int main(){ 81 int n,m; 82 while(scanf("%d%d",&n,&m)!=EOF&&(n!=0||m!=0)){ 83 init(); 84 for(int i=0;i<m;i++){ 85 int u,v; 86 scanf("%d%d",&u,&v); 87 addEdge(u,v); 88 } 89 solve(n); 90 } 91 92 }