Sort Letters by Case

Given a string which contains only letters. Sort it by lower case first and upper case second.

Example

For "abAcD", a reasonable answer is "acbAD"

Note

It's not necessary to keep the original order of lower-case letters and upper case letters.

Challenge

Do it in one-pass and in-place.

Solution

    /** 
     *@param chars: The letter array you should sort by Case
     *@return: void
     */
    public void sortLetters(char[] chars) {
        if(chars == null || chars.length == 0)
            return;
        int lower = 0;
        for(int i = 0; i < chars.length; i++) {
            if(chars[i] - 'a' >= 0 && chars[i] - 'a' <= 26) {
                if(i > lower) {
                    char tmp = chars[i];
                    chars[i--] = chars[lower];
                    chars[lower] = tmp;
                }
                lower++;
            }
        }
    }