Reverse characters in Java while keeping some in place

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 开发者_如何学Csay I want all letters to reverse while all numbers stay in place)?

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).

It sounds like you want something like myReverse("a1b2c3d") to be "d1c2b3a".

Use two indices, one going from each end of the string, looking for swappable characters.

Here's some pseudocode:

i1 = 0; i2 = len - 1;
while (i1 < i2) {
  while (isCharacterThatShouldNotBeSwapped(str.charAt(i1)) && i1 < i2)
    i1++;
  while (isCharacterThatShouldNotBeSwapped(str.charAt(i2)) && i1 < i2)
    i2--;
  if (i1 < i2)
    swapChars(str, i1, i2); /* swap characters at positions i1 and i2 in str */
  i1++; i2--;
}