int[]로 주어질 때

Permutation2.java

List<List<Integer>> permutation(int[] n){
    List<List<Integer>> ret = new ArrayList<>();
    permutation(new ArrayList<>(), n, ret);
    System.out.println(ret.size());
    return ret;
}

void permutation(List<Integer> prefix, int[] n, List<List<Integer>> result){
    if(n.length==0){
        result.add(prefix);
    } else {
        for(int k=0; k<n.length; k++){
            var tmp = new ArrayList<Integer>(prefix);
            tmp.add(n[k]);
            int[] next = Stream.concat(Arrays.stream(Arrays.copyOfRange(n, 0, k)).boxed(),
                    Arrays.stream(Arrays.copyOfRange(n, k+1, n.length)).boxed())
                    .mapToInt(i->i).toArray();

            permutation(tmp, next, result);
        }
    }
}