poj 1304

#include<cstdio>
#include<cstring>
using namespace std;
#define N 27
struct node{
    int in,out;
}degree[N];
int fa[N],rank[N],mem[N],vis[N],top,t,n; 
char str[1001];
int find(int x){
    return fa[x]==x?x:fa[x]=find(fa[x]);
}
int main(){
    scanf("%d",&t);
    while(t--){
        for(int i=1;i<=26;i++){
            fa[i]=i,rank[i]=0;
        }
        memset(degree,0,sizeof degree);
        memset(vis,0,sizeof vis);
        top=0;
        scanf("%d",&n);
        for(int i=1;i<=n;i++){
            memset(str,0,sizeof str);
            scanf("%s",str);
            int a=str[0]-‘a‘+1,b=str[strlen(str)-1]-‘a‘+1;
            if(!vis[a]){
                vis[a]=1;
                mem[++top]=a;
            }
            if(!vis[b]){
                vis[b]=1;
                mem[++top]=b;
            }
            degree[a].out++;degree[b].in++;
            a=find(a);b=find(b);
            if(a!=b){
                if(rank[a]<rank[b]) fa[a]=b;
                else{
                    fa[b]=a;
                    if(rank[a]==rank[b]) rank[a]++;
                }
            }    
        }
        int tmp=find(mem[1]),flag=0;
        for(int i=2;i<=top;i++){
            if(find(mem[i])!=tmp){
                flag=1;break;
            }
        }
        if(flag){
            printf("The door cannot be opened.\n");
            continue;
        }
        int sum=0,flag1=0,flag2=0,ok=1;
        for(int i=1;i<=top&&sum<=2&&ok;i++){
            if(degree[mem[i]].in!=degree[mem[i]].out){
                sum++;
                if(degree[mem[i]].in==degree[mem[i]].out+1) flag1++;
                else if(degree[mem[i]].in==degree[mem[i]].out-1) flag2++;
                else ok=0;
            }    
        }
         if(ok){
            if(flag1==1&&flag2==1 || flag1==0&&flag2==0) 
               printf("Ordering is possible.\n");
            else 
               printf("The door cannot be opened.\n");
         }
         else 
            printf("The door cannot be opened.\n");       
    }
    return 0;
}