import java.util.ArrayList;import java.util.List;
/** * leetcode 78 子集 * https://leetcode-cn.com/problems/subsets/ */public class DayCode {    public static void main(String[] args) {        int[] nums = new int[]{1,2,3};        List<List<Integer>> ans = new DayCode().subSets(nums);    }
    public List<List<Integer>> subSets(int[] nums) {        List<List<Integer>> ans = new ArrayList<>();        if (nums == null) {            return ans;        }        List<Integer> item = new ArrayList<>();        dfs(nums, 0, ans, item);        return ans;    }
    private void dfs(int[] nums, int index, List<List<Integer>> ans, List<Integer> item) {        if (index == nums.length) {            ans.add(new ArrayList<>(item));            return;        }
        dfs(nums, index + 1, ans, item);        item.add(nums[index]);        dfs(nums, index + 1, ans, item);        item.remove(item.size()-1);    }}