<span>POJ2337 Catenyms</span>

#include <iostream>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <cstdio>
#include <string>using namespace std;
struct Edge
{
    int to,next;
    int index;
    bool flag;
}edge[2010];
int head[30],tot;
void init()
{
    tot = 0;
    memset(head,-1,sizeof(head));
}
void addedge(int u,int v,int index)
{
    edge[tot].to = v;
    edge[tot].next = head[u];
    edge[tot].index = index;
    edge[tot].flag = false;
    head[u] = tot++;
}
string str[1010];
int in[30],out[30];
int cnt;
int ans[1010];
void dfs(int u)
{
    for(int i = head[u] ;i != -1;i = edge[i].next)
        if(!edge[i].flag)
        {
            edge[i].flag = true;
            dfs(edge[i].to);
            ans[cnt++] = edge[i].index;
        }
}
int main()
{
    freopen("in.txt","r",stdin);
    int T,n;
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d",&n);
        for(int i = 0;i < n;i++)
            cin>>str[i];
        sort(str,str+n);//要输出字典序最小的解，先按照字典序排序
        init();
        memset(in,0,sizeof(in));
        memset(out,0,sizeof(out));
        int start = 100;
        for(int i = n-1;i >= 0;i--)//字典序大的先加入
        {
            int u = str[i][0] - 'a';
            int v = str[i][str[i].length() - 1] - 'a';
            addedge(u,v,i);
            out[u]++;
            in[v]++;
            if(u < start)start = u;
            if(v < start)start = v;
        }
        int cc1 = 0, cc2 = 0;
        for(int i = 0;i < 26;i++)
        {
            if(out[i] - in[i] == 1)
            {
                cc1++;
                start = i;//如果有一个出度比入度大1的点，就从这个点出发，否则从最小的点出发
            }
            else if(out[i] - in[i] == -1)
                cc2++;
            else if(out[i] - in[i] != 0)
                cc1 = 3;
        }
        if(! ( (cc1 == 0 && cc2 == 0) || (cc1 == 1 && cc2 == 1) ))
        {
            printf("***\n");
            continue;
        }
        cnt = 0;
        dfs(start);
        if(cnt != n)//判断是否连通
        {
            printf("***\n");
            continue;
        }
        for(int i = cnt-1; i >= 0;i--)
        {
            cout<<str[ans[i]];
            if(i > 0)printf(".");
            else printf("\n");
        }
    }
    return 0;
}