Subsets

Given a set of distinct integers, nums, return all possible subsets.

Note:

• Elements in a subset must be in non-descending order.
• The solution set must not contain duplicate subsets.

For example,
If nums = [1,2,3], a solution is:

[
    [3],
    [1],
    [2],
    [1,2,3],
    [1,3],
    [2,3],
    [1,2],
    []
]

Java

public class Solution {
    public List<List<Integer>> subsets(int[] nums) {
        if(nums == null) {
            return null;
        }

        List<List<Integer>> sols = new ArrayList<List<Integer>>();
        sols.add(new ArrayList<Integer>());
        Arrays.sort(nums);
        helper(nums, 0, new ArrayList<Integer>(), sols);
        return sols;
    }

    private void helper(int[] nums, int start, List<Integer> sol, List<List<Integer>> sols) {
        for(int i = start; i < nums.length; i++) {
            sol.add(nums[i]);
            sols.add(new ArrayList<Integer>(sol));
            helper(nums, i + 1, sol, sols);
            sol.remove(sol.size() - 1);
        }
    }
}