Lintcode: Sort Letters by Case

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

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

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

Challenge
Do it in one-pass and in-place.

Two Pointers:

public class Solution {
    /** 
     *@param chars: The letter array you should sort by Case
     *@return: void
     */
    public void sortLetters(char[] chars) {
        //write your code here
        if (chars==null || chars.length==0) return;
        int l=0, r=chars.length-1;
        while (true) {
            while (l<r && isLower(chars, l)) {
                l++;
            }
            while (l<r && isUpper(chars, r)) {
                r--;
            }
            if (l == r) break;
            swap(chars, l, r);
        }
    }
    
    public boolean isLower(char[] chars, int index) {
        if (chars[index]>='a' && chars[index]<='z') return true;
        else return false;
    }
    
    public boolean isUpper(char[] chars, int index) {
        if (chars[index]>='A' && chars[index]<='Z') return true;
        else return false;
    }
    
    public void swap(char[] chars, int l, int r) {
        char temp = chars[l];
        chars[l] = chars[r];
        chars[r] = temp;
    }
}