package lambdasinaction.chap13;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
public class SubsetsMain {
public static void main(String[] args) {
List<List<Integer>> subs = subsets(Arrays.asList(1, 4, 9));
subs.forEach(System.out::println);
}
public static List<List<Integer>> subsets(List<Integer> l) {
if (l.isEmpty()) {
List<List<Integer>> ans = new ArrayList<>();
ans.add(Collections.emptyList());
return ans;
}
Integer first = l.get(0);
List<Integer> rest = l.subList(1,l.size());
List<List<Integer>> subans = subsets(rest);
List<List<Integer>> subans2 = insertAll(first, subans);
return concat(subans, subans2);
}
public static List<List<Integer>> insertAll(Integer first, List<List<Integer>> lists) {
List<List<Integer>> result = new ArrayList<>();
for (List<Integer> l : lists) {
List<Integer> copyList = new ArrayList<>();
copyList.add(first);
copyList.addAll(l);
result.add(copyList);
}
return result;
}
static List<List<Integer>> concat(List<List<Integer>> a, List<List<Integer>> b) {
List<List<Integer>> r = new ArrayList<>(a);
r.addAll(b);
return r;
}
}