子集

import java.util.*;


public class  {

    
     比如数组长度是3 [1,2,3]
     000     []
     001     [3]
     010     [2]
     011     [2,3]
     111     [1,2,3]
     100     [1]
     101     [1,3]
     110     [1,2]
     */

    public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();

        for(int mask = 0; mask < (1 << nums.length); mask++) {
            List<Integer> tmp = new ArrayList<>();
            for(int i=0; i < nums.length; i++) {
                if( ((mask >> i) & 1) == 1) {
                    tmp.add(nums[i]);
                }
            }
            res.add(tmp);
        }
        return res;
    }

    List<List<Integer>> res;
    int mark[] ;
    public List<List<Integer>> subsets_recur(int[] nums) {
        if(nums==null || nums.length==0)
            return null;
        res = new ArrayList<>();
        mark = new int[nums.length];
        dfs(nums,0);
        return res;
    }

    public void dfs(int[] nums, int pos) {
        if(pos == nums.length) {
            ArrayList<Integer> tmp = new ArrayList<>();
            for(int i = 0; i< nums.length; i++) {
                if(mark[i] == 1) {
                    tmp.add(nums[i]);
                }
            }
            res.add(tmp);
            return;
        }
        // 选择pos位的元素
        mark[pos] = 1;
        dfs(nums,pos+1);
        mark[pos] = 0;
        // 不选择
        dfs(nums,pos+1);
    }
    public static void main(String[] args) {
        int[] nums = {1,2,3};
    }
}