var a,x,y,sa,rank,height,wc,wd,ans,flag,b:array[..]of longint;

     ch:array[..]of ansistring;

     n,n1,l,r,mid,last,i,j,m,len:longint;

 procedure swap(var x,y:longint);

 var t:longint;

 begin

  t:=x; x:=y; y:=t;

 end;

 function cmp(a,b,l:longint):boolean;

 begin

  exit((y[a]=y[b])and(y[a+l]=y[b+l]));

 end;

 procedure getsa(n:longint);

 var i,j,p:longint;

 begin

  for i:= to n- do

  begin

   x[i]:=a[i];

   inc(wc[x[i]]);

  end;

  for i:= to m- do wc[i]:=wc[i-]+wc[i];

  for i:=n- downto  do

  begin

   dec(wc[x[i]]);

   sa[wc[x[i]]]:=i;

  end;

  j:=; p:=;

  while p<n do

  begin

   p:=;

   for i:=n-j to n- do

   begin

    y[p]:=i; inc(p);

   end;

   for i:= to n- do

    if sa[i]>=j then begin y[p]:=sa[i]-j; inc(p); end;

   for i:= to n- do wd[i]:=x[y[i]];

   for i:= to m- do wc[i]:=;

   for i:= to n- do inc(wc[wd[i]]);

   for i:= to m- do wc[i]:=wc[i-]+wc[i];

   for i:=n- downto  do

   begin

    dec(wc[wd[i]]);

    sa[wc[wd[i]]]:=y[i];

   end;

   for i:= to n do swap(x[i],y[i]);

   p:=; x[sa[]]:=;

   for i:= to n- do

    if cmp(sa[i-],sa[i],j) then x[sa[i]]:=p-

     else begin x[sa[i]]:=p; inc(p); end;

   j:=j*;

   m:=p;

  end;

 end;

 procedure getheight(n:longint);

 var i,j,k:longint;

 begin

  for i:= to n do rank[sa[i]]:=i;

  k:=;

  for i:= to n- do

  begin

   if k> then dec(k);

   j:=sa[rank[i]-];

   while a[i+k]=a[j+k] do inc(k);

   height[rank[i]]:=k;

  end;

 end;

 function isok(x:longint):boolean;

 var i,j,cnt,s:longint;

 begin

  for i:= to n1 do flag[i]:=;

  cnt:=; s:=;

  for i:= to n do

   if height[i]>=x then

   begin

    cnt:=cnt+flag[b[sa[i]]]; flag[b[sa[i]]]:=;

    cnt:=cnt+flag[b[sa[i-]]]; flag[b[sa[i-]]]:=;

   end

    else

    begin

     if cnt>n1 div  then begin inc(s); ans[s]:=sa[i-]; end;

     cnt:=;

     for j:= to n1 do flag[j]:=;

    end;

  if cnt>n1 div  then begin inc(s); ans[s]:=sa[n]; end;

  if s> then

  begin

   ans[]:=s;

   exit(true);

  end;

  exit(false);

 end;

 procedure init;

 var i:longint;

 begin

  {for i:=0 to n+1 do

  begin

   a[i]:=0; x[i]:=0; y[i]:=0;

   sa[i]:=0; rank[i]:=0; ans[i]:=0;

   wc[i]:=0; wd[i]:=0; b[i]:=0;

   height[i]:=0;

  end;             }

  fillchar(a,sizeof(a),);

  fillchar(b,sizeof(b),);

  fillchar(x,sizeof(x),);

  fillchar(y,sizeof(y),);

  fillchar(sa,sizeof(sa),);

  fillchar(rank,sizeof(rank),);

  fillchar(ans,sizeof(ans),);

  fillchar(wc,sizeof(wc),);

  fillchar(wd,sizeof(wd),);

  fillchar(height,sizeof(height),);

 end;

 begin

  assign(input,'poj3294.in'); reset(input);

  assign(output,'poj3294.out'); rewrite(output);

  while not eof do

  begin

   init;

   readln(n1);

   if n1= then break;

   n:=;

   for i:= to n1 do

   begin

    readln(ch[i]);

    len:=length(ch[i]);

    for j:=n to n+len- do

    begin

     a[j]:=ord(ch[i][j-n+])+;

     b[j]:=i;

    end;

    n:=n+len; a[n]:=i;

    inc(n);

   end;

   dec(n); a[n]:=; m:=;

   getsa(n+);

   getheight(n);

   l:=; r:=n; last:=;

   while l<=r do

   begin

    mid:=(l+r)>>;

    if isok(mid) then begin last:=mid; l:=mid+; end

     else r:=mid-;

   end;

   if last= then writeln('?')

    else

    begin

     for i:= to ans[] do

     begin

      for j:=ans[i] to ans[i]+last- do write(chr(a[j]-));

      writeln;

     end;

    end;

   writeln;

  end;

  close(input);

  close(output);

 end.