Subset with duplication

SubWithDup.java

public class Solution {
    public ArrayList<ArrayList<Integer>> subsetsWithDup(int[] num) {
        Arrays.sort(num);
        final ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if (num.length == 0) {
            result.add(new ArrayList<Integer>());
            return result;
        }
        btrack(num, new boolean[num.length], 0, result);
        return result;        
    }
    
    private static void btrack(int[] num, boolean[] buffer, int k, ArrayList<ArrayList<Integer>> result) {
        if (k == num.length) {
            ArrayList<Integer> ai = new ArrayList<Integer>();
            for (int i=0; i<buffer.length; ++i) {
                if (buffer[i]) ai.add(num[i]);
            }
            result.add(ai);
        } else {
            int start = k;
            int end = k+1;
            buffer[start] = false;
            while (end<buffer.length && num[end] == num[end-1]) {
                buffer[end++] = false;
            }
            btrack(num, buffer, end, result);
            for (; start < end; start++) {
                buffer[start] = true;
                btrack(num, buffer, end, result);
            }
        }
    }
}