之前的求助贴被删了，再发一次

https://www.luogu.com.cn/record/121462279

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int F=10020;

struct sl1{
    int poi,y;
}sack[F];

int n,tou,wei,top,flag,bg,ed,top1,tot;
int rd[F],cd[F],v[F],v1[30],out[F],f[F];
string s[F];
vector<sl1> hav[30];

void fst(){
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
}

bool cmp(string a,string b){
    return a<b;
}

int get(int x){
    if(f[x]==x) return x;
    return f[x]=get(f[x]);
}

void merge(int x,int y){
    f[get(x)]=get(y);
}

void badend(){  
    cout<<"***"<<'\n';
    exit(0);
}

void dfs(int x){
    if(top==n){
        for(int i=1;i<=n;++i){
            if(i==n){
                int len=s[out[i]].length();
                for(int j=0;j<len-1;j++) cout<<s[out[i]][j];
            }
            else cout<<s[out[i]];
        }
        exit(0);
    }
    for(int i=0;i<hav[x].size();++i){
        sl1 y=hav[x][i];
        if(!v1[y.y]){
            v1[y.y]=1;
            out[++top]=y.y;
            dfs(y.poi);
            v1[y.y]=0;
            --top;
        }
    }
}

signed main(){
    freopen("try.in","r",stdin);
    fst();
    cin>>n;
    for(int i=1;i<=n;++i){
        cin>>s[i];
        s[i]+=".";
    }
    sort(s+1,s+1+n);
    for(int i=1;i<=26;++i) f[i]=i;
    for(int i=1;i<=n;++i){
        int len=s[i].length()-1;
        tou=s[i][0]-'a'+1;
        wei=s[i][len-1]-'a'+1;
        v[tou]=v[wei]=1;
        if(get(tou)!=get(wei)) merge(tou,wei);
        rd[wei]++;
        cd[tou]++;
        hav[tou].push_back({wei,i});
    }
    for(int i=1;i<=26;++i){
        if(v[i]&&f[i]==i) flag++;
    }
    if(flag>1) badend();
    for(int i=1;i<=26;++i){
        if(v[i]){
            if(cd[i]==rd[i]+1){
                if(bg) badend();
                bg=i;
            }
            else{
                if(rd[i]==cd[i]+1){
                    if(ed) badend();
                    ed=i;
                }
                else if(rd[i]!=cd[i]) badend();
            }
        }
    }
    if((bg&&!ed)||(!bg&&ed)) badend();
    if(!bg) bg=s[1][0]-'a'+1;
    dfs(bg);
    return 0;
}