表達式轉化

        由於noip居然可能考這種東西，於是很不情願的來回憶這道字符串神題。

              後綴表達式的運算裸做就行了，而轉後綴的時候只要把運算符之間的優先級推出即可，if流，反正感覺比打表好看些。

         這種東西直接貼 代碼算了：

      

program lmd;
const
       snum='0123456789';
var
   st:array[0..1000]of char;
   num:array[0..1000]of real;
   top,i,len:longint;
   x,y:real;
   s,ss,t:string;
procedure inf;
begin
   assign(input,'Expression.in');
   assign(output,'Expression.out');
   reset(input);rewrite(output);
end;
procedure ouf;
begin
   close(input);close(output);
end;
procedure init;
begin
    readln(s); s:=s+'@';
end;
procedure bug;
begin
    write('wrong input');
    ouf; halt;
end;
function use(x,y:char):longint;
begin
   if (x='+') or (x='-') then
     begin
       if (y='*') or (y='/') or(y='(') then exit(0);
       {if (x='+') or (x='-') or(x=')') or (x='@') then }exit(1);
     end;
   if (x='*') or (x='/') then
     begin
       if (y='(') then exit(0);
       exit(1);
     end;
   if x='(' then
     begin
       if y=')' then exit(2);
       if y='@' then bug;
       exit(0);
     end;
   if x=')' then
     begin
        if x='(' then bug;
        exit(1);
     end;
   if x='@' then
      begin
        if y='@' then exit(2);
        exit(0);
      end;
end;
procedure prepare;
begin
    top:=1; st[1]:='@';
    i:=1; len:=length(s);
    while i<=len do
      begin
         if pos(s[i],snum)>0 then
           begin
              ss:=s[i];
              while  pos(s[i+1],snum)>0 do
                begin
                  inc(i); ss:=ss+s[i];
                end;
              t:=t+ss+' ';
           end
         else
           begin
              while use(st[top],s[i])=1 do
                begin
                   t:=t+st[top]; dec(top);
                end;
              if use(st[top],s[i])=2 then
                  begin
                     st[top]:=' ';dec(top);
                  end
              else
                 begin
                    inc(top); st[top]:=s[i];
                 end;
           end;
         inc(i);
      end;
    writeln(t);
end;
procedure work;
begin
    i:=1; len:=length(t); top:=0;
    while i<=len do
     begin
        if pos(t[i],snum)>0 then
          begin
            x:=ord(t[i])-ord('0');
            while pos(t[i+1],snum)>0  do
              begin
                 inc(i);  x:=x*10+ord(t[i])-ord('0');
              end;
            inc(top); num[top]:=x;
            inc(i);
         end
        else
          begin
             x:=num[top-1]; y:=num[top]; dec(top);
             case t[i] of
                '+':num[top]:=x+y;
                '-':num[top]:=x-y;
                '*':num[top]:=x*y;
                '/':num[top]:=x/y;
                end;
          end;
        inc(i);
     end;
    write(num[top]:0:5);
end;
begin
    inf;
    init;
    prepare;
    work;
    ouf;
end.