代码如下，实在想不出来。

#include <bits/stdc++.h>

using namespace std;
vector<string> a;
vector<int> v;
vector<vector<pair<int, string>>> m(26);
vector<int> d(26);
int total;
string trace;

void dfs(int n)
{
    if(total == int(a.size()-1)) {
        cout << trace+"."+a[n] << endl;
        exit(0);
    } else {
        string tmp = trace;
        if(trace == "") trace = a[n];
        else trace += "."+a[n];

        int t = a[n][a[n].size()-1]-'a';
        for(int i = 0; i < m[t].size(); i ++) {
            if(v[m[t][i].first] != 1) {
                v[m[t][i].first] = 1;
                total ++;
                dfs(m[t][i].first);
                total --;
                v[m[t][i].first] = 0;
            }
        }
        trace = tmp;
    }
}

bool cmp(pair<int, string> & a, pair<int, string> & b)
{
    return a.second < b.second;
}
int main ( int argc, char *argv[] )
{
    int N;
    cin >> N;

    a.resize(N);
    v.resize(N);

    for(int i = 0; i < N; i ++) {
        cin >> a[i];

        d[a[i][0]-'a'] ++;
        d[a[i][a[i].size()-1]-'a'] ++;
    }

    sort(a.begin(), a.end());

    int odd = 0;
    int c = 0;
    for(int i = 0; i < 26; i ++) {
        if(d[i]%2 != 0) {
            if(odd == 0) c = i;
            odd ++;
        }
    }

    if(odd != 0 && odd != 2) {
        cout << "***" << endl;
        return 0;
    }

    int mins = -1;
    for(int i = 0; i < N; i ++) {
        if(odd != 0 && mins == -1 && a[i][0] == 'a'+c) mins = i;
        m[a[i][0]-'a'].push_back({i, a[i]});
    }

    if(mins == -1) mins = 0;
    v[mins] = 1;
    dfs(mins);

    cout << "***" << endl;

    return 0;
}               /* ----------  end of function main  ---------- */