LintCode 子集题目代码

class Solution {
    /**
     * @param S: A set of numbers.
     * @return: A list of lists. All valid subsets.
     */
    public ArrayList<ArrayList<Integer>> subsets(int[] num) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        
        if(num == null || num.length == 0)
            return res;
        
        ArrayList<Integer> list = new ArrayList<>();
        
        Arrays.sort(num);
        
        helper(res, list, num, 0);
        return res;
    }

    private void helper(ArrayList<ArrayList<Integer>> res, ArrayList<Integer> list, int[] num, int pos) {
        
        res.add(new ArrayList<>(list));
        
        for(int i=pos;i<num.length;i++) {
            list.add(num[i]);
            helper(res, list, num, i+1);
            list.remove(list.size() - 1);
        }
        
    }
}