정보처리기사 실기 · 2024년 2회 · 13번

public static String calculFn(
        String str, int index, boolean[] seen) {
    if(index < 0) return "";
    char c = str.charAt(index);
    String result = calculFn(str, index-1, seen);
    if(!seen[c]) {
        seen[c] = true;
        return c + result;
    }
    return result;
}
public static void main(String[] args) {
    String str = "abacabcd";
    boolean[] seen = new boolean[256];
    System.out.print(
        calculFn(str, str.length()-1, seen));
}