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import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class AllSubList {
    
    /**
     * 1.get all subsets of {2,3} as A
     * 2.all subsets of {2,3} adds 1 as B 
     * 3.A+B = all subsets of {1,2,3}
     * @param args
     */
    public static void main(String[] args) {
        System.out.println(subsets(Arrays.asList(1,2,3)));
    }


    static List<List<Integer>> subsets(List<Integer> list){
        if(list.isEmpty()){
            List<List<Integer>> ans = new ArrayList<>();
            ans.add(Collections.emptyList());
            return ans;
        }

        Integer first = list.get(0);
        List<Integer> rest = list.subList(1,list.size());

        List<List<Integer>> subans = subsets(rest);
        List<List<Integer>> subans2 = insertAll(first,subans);

        return concat(subans,subans2);
    }

    //referential transparency
    private static List<List<Integer>> concat(List<List<Integer>> subans, List<List<Integer>> subans2) {
        List<List<Integer>> r = new ArrayList<>(subans);
        r.addAll(subans2);
        return r;
    }

    //referential transparency
    private static List<List<Integer>> insertAll(Integer first, List<List<Integer>> subans) {
        List<List<Integer>> result = new ArrayList<>();
        for(List<Integer> list:subans){
            List<Integer> copyList = new ArrayList<>();
            copyList.add(first);
            copyList.addAll(list);
            result.add(copyList);
        }
        return result;
    }
}