下列程序的功能是输入一个长度为20的字符串，先输出，再将字符串间所有空格删除后再输出。

   program ex;
   Const n=20;
   var a:array[1..20]of char;
   i,j,k:integer;ch:char;
begin
     writeln;
     for i:=1 to n do_________
     for j:=1 to 20 do write(a[j]:2);
     i:=1;while____________________________do i:=i+1;
     j:=j+1;
     while j<=n do
       begin
         while (j<=n)and (a[j]='')do__________________
         if _________________then
            begin
                ch:=a[1];a[1]:=a[j];a[j]:=ch;
            end;
         _________________;
        end;
        for i:=1 to n do write(a[i]);
        writeln;
   end.


共5个空，望高手赐教，不胜感激，救命！！