堆栈以abcd顺序入栈，如何求所有合法的出栈序列

LeeChange:昨天的货箱用了非递归的,今天就用个递归的吧.

program Project2;

{$APPTYPE CONSOLE}

uses

 SysUtils;

type

 TIntegerArr = array [1..4] of Integer;

var

 PopedList: TIntegerArr;

 Stack: TIntegerArr;

procedure TryDoIt(PopedIndex, UnPushIndex, StackTop: Integer; Stack: TIntegerArr);

var

 i: Integer;

begin

 if PopedIndex=5 then

 begin

   for i:=1 to 4 do

     Write(Chr(Ord('a')-1+PopedList[i]), ' ');

   WriteLn;

   Exit

 end;

 if StackTop>0 then

 begin

   PopedList[PopedIndex]:=Stack[StackTop];

   TryDoIt(PopedIndex+1, UnPushIndex, StackTop-1, Stack)

 end;

 if UnPushIndex<=4 then

 begin

   Stack[StackTop+1]:=UnPushIndex;

   TryDoIt(PopedIndex, UnPushIndex+1, StackTop+1, Stack)

 end;

end;

begin

 TryDoIt(1, 1, 0, Stack);

 ReadLn

end.

另外,任何结果字符串的子串也是合法串.