Java - 순열 ( Permutation )

public static void main(String[] args) {
    System.out.println(getPermutations("324"));
}

/**
 *
 * 순열 구하기
 */
public static List<String> getPermutations(String s){
    if(s==null) return null;
    //boolean 은 누구를 선택 했는지 판별 파라미터
    return permuRec(s,new boolean[s.length()],"",new ArrayList<String>());
}
private static List<String> permuRec(String s, boolean[] pick,String perm,ArrayList<String> result){
    //종료 조건
    if(perm.length() == s.length()){
        result.add(perm);
        return result;
    }
    for(int i=0;i<s.length();i++){
        if(pick[i]) continue;
        pick[i] = true;
        permuRec(s,pick,perm + s.charAt(i),result);
        pick[i] = false;
    }
    return result;
}