题意：判断有向图是否为树
链接：点我
这题用并查集判断连通，连通后有且仅有1个入度为0，其余入度为1，就是树了
1 #include<cstdio> 2 #include<iostream> 3 #include<algorithm> 4 #include<cstring> 5 #include<cmath> 6 #include<queue> 7 #include<map> 8 using namespace std; 9 #define MOD 1000000007 10 const int INF=0x3f3f3f3f; 11 const double eps=1e-5; 12 #define cl(a) memset(a,0,sizeof(a)) 13 #define ts printf("***** "); 14 const int MAXN=1000000; 15 int n,m,tt; 16 int a[MAXN]; 17 int f[MAXN]; 18 int in[MAXN]; 19 bool vis[MAXN]; 20 int find(int x) 21 { 22 if(f[x]==-1) return x; 23 else return f[x]=find(f[x]); 24 } 25 void bing(int x,int y) 26 { 27 int t1=find(x); 28 int t2=find(y); 29 if(t1!=t2) 30 f[t1]=t2; 31 } 32 int main() 33 { 34 int i,j,k; 35 #ifndef ONLINE_JUDGE 36 freopen("1.in","r",stdin); 37 #endif 38 int cnt=0; 39 int u,v; 40 int iCase=0; 41 bool flag=true; 42 memset(f,-1,sizeof(f)); 43 memset(vis,false,sizeof(vis)); 44 memset(in,0,sizeof(in)); 45 while(scanf("%d%d",&u,&v)!=EOF) 46 { 47 if(u==-1&&v==-1) break; 48 if(u==0&&v==0) 49 { 50 iCase++; 51 printf("Case %d ",iCase); 52 int t0=0; 53 for(i=0;i<cnt;i++) 54 { 55 if(find(a[i])!=find(a[0])) 56 { 57 flag=0; 58 break; 59 } 60 if(in[a[i]]==0) t0++; 61 else if(in[a[i]]>1) 62 { 63 flag=0; 64 break; 65 } 66 } 67 if(t0!=1)flag=false; 68 if(cnt==0)flag=true; 69 if(flag)printf("is a tree. "); 70 else printf("is not a tree. "); 71 cnt=0; 72 memset(f,-1,sizeof(f)); 73 memset(vis,false,sizeof(vis)); 74 memset(in,0,sizeof(in)); 75 flag=true; 76 } 77 else 78 { 79 if(!vis[u]) 80 { 81 vis[u]=1; 82 a[cnt++]=u; 83 } 84 if(!vis[v]) 85 { 86 vis[v]=1; 87 a[cnt++]=v; 88 } 89 if(find(u)==find(v)) flag=0; 90 else 91 { 92 bing(u,v); 93 in[v]++; 94 } 95 } 96 } 97 }