UVa-10129 Play on Words

#include <bits/stdc++.h>

const int maxn = 28;
using namespace std;

int G[maxn][maxn];
int tmpG[maxn][maxn];
int known[maxn];
void DFS(int s)
{
    for(int u=0; u<maxn; u++)
    {
        if(!known[u]&&tmpG[s][u])
        {
            known[u] = 1;
            DFS(u);
        }
    }
    return ;
}

bool bing(int vnum)
{
    int count = 0;
    DFS(0);
    for(int i = 0;i < maxn;i ++)
    {
        if(known[i]>0)
            count ++;
    }
    return count==vnum;
}

int main()
{
    int T;
    cin >> T;
    while(T --)
    {
        memset(G,0,sizeof(G));
        memset(tmpG,0,sizeof(tmpG));
        memset(known,0,sizeof(known));
        int vnum = 0;
        int v[maxn] {0};
        int n;
        cin >> n;

        int out[maxn] {0};
        int in[maxn] {0};

        vector<int> diff;
        while(n --)
        {
            string tmp;
            cin >> tmp;
            G[tmp[0]-'a'][tmp[tmp.size()-1]-'a'] ++;
            tmpG[tmp[0]-'a'][tmp[tmp.size()-1]-'a'] = 1;
            tmpG[tmp[tmp.size()-1]-'a'][tmp[0]-'a'] = 1;
            if(v[tmp[0]-'a']==0)
            {
                v[tmp[0]-'a'] = 1;
                vnum ++;
            }
            if(v[tmp[tmp.size()-1]-'a']==0)
            {
                v[tmp[tmp.size()-1]-'a'] = 1;
                vnum ++;
            }
        }

        if(!bing(vnum))
        {
            cout << "The door cannot be opened." << endl;
            continue;
        }

        for(int i = 0; i < maxn; i ++)
        {
            int sum = 0;
            for(int j = 0; j < maxn; j ++)
            {
                sum += G[i][j];
            }
            in[i] = sum;
        }

        for(int i = 0; i < maxn; i ++)
        {
            int sum = 0;
            for(int j = 0; j < maxn; j ++)
            {
                sum += G[j][i];
            }
            out[i] = sum;
        }

        for(int i = 0; i < maxn; i ++)
        {
            if(in[i]!=out[i])
                diff.push_back(in[i]-out[i]);
        }

        if(diff.size()==0)
        {
            cout << "Ordering is possible." << endl;
            continue;
        }
        else if(diff.size()==2)
        {
            if(diff[0]==-1&&diff[1]==1||diff[0]==1&&diff[1]==-1)
            {
                cout << "Ordering is possible." << endl;
                continue;
            }
        }
        cout << "The door cannot be opened." << endl;
    }
    return 0;
}