program ReverseStringLetters;
// Compiler directives: modern Object Pascal mode and dynamic long strings enabled
{$mode objfpc}{$H+}
{
Checks if a character is an ASCII letter using Pascal's efficient
set membership operator.
}
function IsAlpha(C: Char): Boolean;
begin
Result := C in ['a'..'z', 'A'..'Z'];
end;
{
Reverses only the alphabetic characters in a string in-place,
leaving special characters, digits, and spaces in their original positions.
@param AText Reference to the string to modify in-place.
}
procedure ReverseOnlyLetters(var AText: string);
var
Left, Right: Integer;
Temp: Char;
begin
if Length(AText) <= 1 then
Exit; // Guard against empty or single-character strings
Left := 1; // Free Pascal strings are 1-indexed by default
Right := Length(AText);
// Use a two-pointer approach moving inward from both ends
while Left < Right do
begin
// Advance left pointer until it points to an alphabetic character
while (Left < Right) and not IsAlpha(AText[Left]) do
Inc(Left);
// Move right pointer backward until it points to an alphabetic character
while (Left < Right) and not IsAlpha(AText[Right]) do
Dec(Right);
// Swap the valid letters found at both pointers
if Left < Right then
begin
Temp := AText[Left];
AText[Left] := AText[Right];
AText[Right] := Temp;
Inc(Left);
Dec(Right);
end;
end;
end;
var
MessageText: string;
begin
// Input string specified
MessageText := 'ab*#cde!@$,fg{}';
Writeln('Original: ', MessageText);
// Apply the reversal function in-place
ReverseOnlyLetters(MessageText);
Writeln('Reversed: ', MessageText);
end.
(*
run:
Original: ab*#cde!@$,fg{}
Reversed: gf*#edc!@$,ba{}
*)
