在java中使用递归方法返回有序字符串？

public class RecursiveSort {
    public static String sortString(String str) {
        if (str.length() <= 1) {
            return str;
        } else {
            int mid = str.length() / 2;
            String left = sortString(str.substring(0, mid));
            String right = sortString(str.substring(mid));
            return merge(left, right);
        }
    }

    private static String merge(String left, String right) {
        StringBuilder result = new StringBuilder();
        int i = 0, j = 0;
        while (i < left.length() && j < right.length()) {
            if (left.charAt(i) < right.charAt(j)) {
                result.append(left.charAt(i));
                i++;
            } else {
                result.append(right.charAt(j));
                j++;
            }
        }
        while (i < left.length()) {
            result.append(left.charAt(i));
            i++;
        }
        while (j < right.length()) {
            result.append(right.charAt(j));
            j++;
        }
        return result.toString();
    }

    public static void main(String[] args) {
        String str = "dcba";
        String sortedStr = sortString(str);
        System.out.println(sortedStr);  // 输出：abcd
    }
}