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;
     }
 }