Algorithm question: reversing string with special characters

Given a string, that contains special character together with alphabets (‘a’ to ‘z’ and ‘A’ to ‘Z’), reverse the string such a way that special characters are not affected.

Example
input "abc-defg" return "gfe-dcba"

String reverse(String input) {
    Stack<Character> result = new Stack<>();
    StringBuilder result2 = new StringBuilder();
    char[] in = input.toCharArray();
    for (char c : in) {
        if (Character.isLetter(c)) {
            result.push(c);
        }
    }
    for (char c : in) {
        if (Character.isLetter(c)) {
            result2.append(result.pop());
        } else {
            result2.append(c);
        }
    }

    return result2.toString();
}