Latest version

Combination.java

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

void combination(List<Integer> prefix, int[] n, int r, List<List<Integer>> result){
    if(r==0) result.add(prefix);
    else {
        for(int k=0; k<=n.length-r; k++){
            ArrayList<Integer> prefixcopy = new ArrayList<>(prefix);
            prefixcopy.add(n[k]);
            
            combination(prefixcopy, Arrays.copyOfRange(n, k+1, n.length), r-1, result);
        }
    }
}