Subclasses

　　Given a collection of numbers, return all possible subclasses.

public class Solution {
    public List<List<Integer>> permute(int[] num) {
        ArrayList<List<Integer>> result = new ArrayList<List<Integer>>();
        ArrayList<Integer> row = null;
        if(num == null ||num.length == 0) return result;
        long size = (long)Math.pow(2, num.length);
        for(long i = 0; i < size; i ++){
            row = new ArrayList<Integer>();
            for(int j = 0; j < num.length; j ++){
                if((i >> j) % 2 == 1) row.add(num[j]);
            }
            result.add(row);
        }
        return result;
    }
}