有一个字符串，每次只能从两端取，求取得的字典序最小的子符序列。

每次比较剩余字符串的正向和反向字符串的大小，正向大就在尾取，否则在头取，相当于贪心了吧。

1 program pku3617(input,output);  2 var  3    s1,s2  : ansistring;  4    answer : ansistring;  5    n      : longint;  6 procedure init;  7 var  8    i  : longint;  9    ch : char; 10 begin 11    readln(n); 12    s1:=''; 13    for i:=1 to n do 14    begin 15       readln(ch); 16       s1:=s1+ch; 17    end; 18    s2:=''; 19    for i:=length(s1) downto 1 do 20       s2:=s2+s1[i]; 21 end; {
      init } 22 procedure main; 23 var 24    i : longint; 25 begin 26    answer:=''; 27    for i:=1 to n do 28    begin 29       if s1>s2 then 30       begin 31      answer:=answer+s2[1]; 32      delete(s2,1,1); 33      delete(s1,length(s1),1); 34       end 35       else 36       begin 37      answer:=answer+s1[1]; 38      delete(s1,1,1); 39      delete(s2,length(s2),1); 40       end; 41    end; 42 end; {
      main } 43 procedure print; 44 var 45    tmp : ansistring; 46 begin 47    while length(answer)>=80 do 48    begin 49       tmp:=copy(answer,1,80); 50       writeln(tmp); 51       delete(answer,1,80); 52    end; 53    if answer<>'' then 54       writeln(answer); 55 end; {
      print } 56 begin 57    init; 58    main; 59    print; 60 end.