UVA 10441 Catenyms 欧拉道路

// #pragma comment(linker, "/STACK:1024000000,1024000000")
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<iostream>
#include<sstream>
#include<cmath>
#include<climits>
#include<string>
#include<map>
#include<queue>
#include<vector>
#include<stack>
#include<set>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define pb(a) push_back(a)
#define INF 0x1f1f1f1f
#define lson idx<<1,l,mid
#define rson idx<<1|1,mid+1,r

void debug()
{
#ifdef ONLINE_JUDGE
#else
    freopen("d:\\in.txt","r",stdin);
    freopen("d:\\out1.txt","w",stdout);
#endif
}
char getch()
{
    char ch;
    while((ch=getchar())!=EOF)
    {
        if(ch!=' '&&ch!='\n')return ch;
    }
    return EOF;
}

struct point
{
    int x,y,dir,col, t;
    point() {}
    point (int a,int b,int c,int d,int e)
    {
        x=a;
        y=b;
        dir=c;
        col=d;
        t=e;
    }
};
int vis[1001];
string dic[1001];
stack<int> sta;
int n;
int f(int u)
{
    for(int i=1;i<=n;i++)if(!vis[i]&&dic[i][0]==u+'a')
    {
        vis[i]=1;
        f(dic[i][dic[i].size()-1]-'a');
        sta.push(i);
    }
    return 0;
}
int da[26];
int cvis[26];
int find(int a)
{
    return a==da[a]?a:da[a]=find(da[a]);
}
int can()
{
    for(int i=0;i<26;i++)da[i]=i;
    memset(cvis,0,sizeof(cvis));
    for(int i=1;i<=n;i++)
    {
        int fa=find(dic[i][0]-'a');
        int fb=find(dic[i][dic[i].size()-1]-'a');
        cvis[dic[i][0]-'a']=1;cvis[dic[i][dic[i].size()-1]-'a']=1;
        if(fa!=fb)da[fa]=fb;
    }
    int num=0;
    for(int i=0;i<26;i++)
        if(da[i]==i&&cvis[i]==1)num++;
    return num==1?1:0;
}
int main()
{
    int t;
    cin>>t;
    for(int ca=1;ca<=t;ca++)
    {

        cin>>n;
        int deg[26]={0};
        for(int i=1;i<=n;i++)
        {
            cin>>dic[i];
        }
        sort(dic+1,dic+1+n);
        for(int i=1;i<=n;i++)
        {
            deg[dic[i][0]-'a']++;deg[dic[i][dic[i].size()-1]-'a']--;
        }
        int in=-1,out=-1;
        int allzero=1;
        for(int i=0;i<26;i++)
        {
            if(deg[i]==1)
            {
                allzero=0;
                if(in!=-1)
                {
                    in=-1;break;
                }
                else in=i;
            }else if(deg[i]==-1)
            {
                allzero=0;
                if(out!=-1)
                {
                    out=-1;break;
                }
                else out=i;
            }else if(deg[i]!=0)
            {
                allzero=0;
                in=-1;out=-1;break;
            }
        }
        if(allzero)
        {
            in=dic[1][0]-'a';out=dic[1][0]-'a';
        }
        if(in==-1||out==-1||!can())
        {
            cout<<"***"<<endl;
            continue;
        }
        memset(vis,0,sizeof(vis));
        while(!sta.empty())sta.pop();
        f(in);
        while(!sta.empty())
        {
            int k=sta.top();sta.pop();
            cout<<dic[k];
            cout<<(char)((sta.empty())?'\n':'.');
        }
    }
    return 0;
}