Question

I have a String and want to reverse it so that some characters get reversed, while others remain in place. How can I implement that (e.g. lets say I want all letters to reverse while all numbers stay in place)?

Answer 1

You can create an automaton for it:

iterate over the string, insert each letter into the stack and replace it with a special character which is not in the original string (i.e. $), do not change digits at all.

iterate over the string again, replace every $ with the head of the stack.

note: since String is immutable you will need to first export it to a StringBuilder or CharSequence

EDIT

for example:

    String inp = "a45string1test";
    char[] cs = new char[inp.length()];
    inp.getChars(0, inp.length(), cs, 0);
    Stack<Character> stack = new Stack<Character>();
    for (int i =0;i<cs.length;i++) {
        if (Character.isDigit(cs[i])) continue;
        stack.push(cs[i]);
        cs[i] = '$';
    }
    for (int i=0;i<cs.length;i++) {
        if (cs[i] == '$') cs[i] = stack.pop();
    }
    System.out.println(cs);

result will be t45setgni1rtsa

complexity is O(n): iterating the string (twice) – since all stack operations are O(1).