Question.java

package Question8_3;

import java.util.*;

public class Question {

	public static ArrayList<ArrayList<Integer>> getSubsets(ArrayList<Integer> set, int index) {
		ArrayList<ArrayList<Integer>> allsubsets;
		if (set.size() == index) {
			allsubsets = new ArrayList<ArrayList<Integer>>();
			allsubsets.add(new ArrayList<Integer>()); // Empty set
		} else {
			allsubsets = getSubsets(set, index + 1);
			int item = set.get(index);
			ArrayList<ArrayList<Integer>> moresubsets = new ArrayList<ArrayList<Integer>>();
			for (ArrayList<Integer> subset : allsubsets) {
				ArrayList<Integer> newsubset = new ArrayList<Integer>();
				newsubset.addAll(subset); //
				newsubset.add(item);
				moresubsets.add(newsubset);
			}
			allsubsets.addAll(moresubsets);
		}
		return allsubsets;
	}
	
	public static ArrayList<ArrayList<Integer>> getSubsets2(ArrayList<Integer> set) {
		ArrayList<ArrayList<Integer>> allsubsets = new ArrayList<ArrayList<Integer>>();
		int max = 1 << set.size(); 
		for (int i = 0; i < max; i++) {
			ArrayList<Integer> subset = new ArrayList<Integer>();
			int k = i;
			int index = 0;
			while (k > 0) {
				if ((k & 1) > 0) subset.add(set.get(index));
				k >>= 1;
				index++;
			}
			allsubsets.add(subset);
		}
		return allsubsets;
	}
	
	public static void main(String[] args) {
		ArrayList<Integer> list = new ArrayList<Integer>();
		for (int i = 0; i < 3; i++) {
			list.add(i);
		}
		ArrayList<ArrayList<Integer>> subsets = getSubsets(list, 0);
		System.out.println(subsets.toString());
		
		ArrayList<ArrayList<Integer>> subsets2 = getSubsets2(list);
		System.out.println(subsets2.toString());		
	}

}