poj3294

#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cstring>
#include <map>
#include <queue>
#include <set>
#include <cassert>
#include <stack>
#include <bitset>
//#include <unordered_set>
#define mkp make_pair
#define err cout<<"here"<<endl
using namespace std;
const double EPS=1e-12;
typedef long long lon;
typedef unsigned long long ull;
typedef map<ull,int>::iterator IT;
const lon SZ=500010,SSZ=120,APB=26,mod=97,one=1;
const lon INF=0x7FFFFFFF;
int n,maxlen[SZ],minlen[SZ],trans[SZ][APB],slink[SZ];
int cnt,in[SZ];
bitset<120> bt[SZ];
struct nd{
    int to,wt;
    nd(int a=0,int b=0):to(a),wt(b){}
};
char ch[SSZ][1010];

int add(int pre,int cur,int num)
{
    if(trans[pre][cur])
    {
        int x=trans[pre][cur];
        if(maxlen[x]==maxlen[pre]+1)
        {
            bt[x].set(num);
            return x;    
        }
        else
        {
            int z=++cnt;
            memcpy(trans[z],trans[x],sizeof(trans[x]));
            maxlen[z]=maxlen[pre]+1;
            slink[z]=slink[x];
            slink[x]=z;
            minlen[z]=maxlen[slink[z]]+1;
            minlen[x]=maxlen[z]+1;
            for(int u=pre;u!=-1&&trans[u][cur]==x;u=slink[u])
            trans[u][cur]=z;
            bt[z].set(num);
            return z;
        }
    }
    int z=++cnt;
    bt[z].set(num);
    int u=pre;
    for(;u!=-1&&!trans[u][cur];u=slink[u])
    {
        trans[u][cur]=z;
    }
    if(u==-1)
    {
        maxlen[z]=maxlen[pre]+1;
        minlen[z]=1;
        slink[z]=0;
    }
    else
    {
        int x=trans[u][cur];
        if(maxlen[x]==maxlen[u]+1)
        {
            maxlen[z]=maxlen[pre]+1;
            minlen[z]=maxlen[x]+1;
            slink[z]=x;
        }
        else
        {
            int y=++cnt;
            memcpy(trans[y],trans[x],sizeof(trans[x]));
            bt[y]=bt[x]|bt[z];
            maxlen[y]=maxlen[u]+1;
            minlen[y]=maxlen[slink[x]]+1;
            slink[y]=slink[x];
            maxlen[x]=maxlen[x];
            minlen[x]=maxlen[y]+1;
            slink[x]=y;
            maxlen[z]=maxlen[pre]+1;
            minlen[z]=maxlen[y]+1;
            slink[z]=y;
            for(;u!=-1&&trans[u][cur]==x;u=slink[u])
            {
                trans[u][cur]=y;
            }
        }
    }
    return z;
}

void topo()
{
    for(int i=1;i<=cnt;++i)
    {
        ++in[slink[i]];
    }
    queue<int> q;
    for(int i=1;i<=cnt;++i)
    {
        if(in[i]==0)
        {
            q.push(i);
        }
    }
    for(;q.size();)
    {
        int fr=q.front();
        q.pop();
        bt[slink[fr]]|=bt[fr];
        --in[slink[fr]];
        if(in[slink[fr]]==0&&slink[fr])
        {
            q.push(slink[fr]);
        }
    }
}

void init()
{
    slink[0]=-1;
    for(int i=1;i<=n;++i)
    {
        cin>>ch[i]+1;
        int pre=0;
        for(int j=1;ch[i][j];++j)
        {
            pre=add(pre,ch[i][j]-‘a‘,i);
        }
    }
    topo();
}

void dfs(int x,int d,int maxv)
{
    if(d==maxv)
    {
        if(bt[x].count()<=n/2)return;
        ch[0][d]=0;
        cout<<ch[0]<<endl;
        return;
    }
    for(int i=0;i<APB;++i)
    {
        int t=trans[x][i];
        if(t&&maxlen[t]==maxlen[x]+1)
        {
            ch[0][d]=‘a‘+i;
            dfs(t,d+1,maxv);
        }
    }
}

void work()
{
    int maxv=0;
    for(int i=0;i<=cnt;++i)
    {
        if(bt[i].count()>n/2)
        {
            maxv=max(maxv,maxlen[i]);
        }
    }
    if(maxv==0)cout<<"?"<<endl;
    else
    {
        dfs(0,0,maxv);
    }
}

void release()
{
    for(int i=0;i<=cnt;++i)
    {
        memset(trans[i],0,sizeof(trans[i]));
        in[i]=0;
        bt[i].reset();
    }
    cnt=0;
}

int main()
{
    std::ios::sync_with_stdio(0);
    //freopen("d:\\1.txt","r",stdin);
    int casenum;
    //cin>>casenum;
    //cout<<casenum<<endl;
    //for(int time=1;time<=casenum;++time)
    for(int time=1;cin>>n,n;++time)
    {
        if(time!=1)cout<<endl;
        init();
        work();//err;
        release();
    }
    return 0;
}