LintCode-49. 字符大小写排序

样例 1:
    输入:  "abAcD"
    输出:  "acbAD"

样例 2:
    输入: "ABC"
    输出:  "ABC"

    public void sortLetters(char[] chars) {
        int i = 0, j = chars.length - 1;
        char a;
        while (i < j) {
            if (chars[i] >= 'a') i++;
            else if (chars[j] <= 'Z') j--;
            else {
                a = chars[i];
                chars[i] = chars[j];
                chars[j] = a;
            }
        }
    }