Sort Letters by Case

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

Notice

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"

Solution: Sort color easy mode.

public void sortLetters(char[] chars) {
    if(chars == null || chars.length == 0) {
        return ;
    }
    int i = 0;
    int left = 0, right = chars.length - 1;
    while(i <= right) {
        if(Character.isLowerCase(chars[left])) {
            char tmp = chars[i];
            chars[i] = chars[left];
            chars[left] = tmp;
            left++;
            i++; 
        }
        else {
            char tmp = chars[i];
            chars[i] = chars[right];
            chars[right] = tmp;
            right--;
        }
    }
}