Sort Letters by Case

Source

• lintcode: 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.

Java

public class Solution {
    /** 
     *@param chars: The letter array you should sort by Case
     *@return: void
     */
    public void sortLetters(char[] chars) {
        int n = chars.length;

        int start=0;
        int end = n-1;

        while(start<=end){
            if(Character.isLowerCase(chars[start])){
                start++;
            }
            else{
                swap(chars, start, end);
                end--;
            }
        }
    }

    public void swap(char[] chars, int start, int end){
        char tmp = chars[start];
        chars[start] = chars[end];
        chars[end] = tmp;
    }
}