回溯-求出数组的所有子序列【学习算法】

import java.util.ArrayList;
import java.util.Scanner;
/*
3
1 2 3
4
1 2 3 4
2
2 1
 */
public class Main {
    public static void main(String[] args) {
        Scanner scanner=new Scanner(System.in);
        int n = scanner.nextInt();
        int[] arr=new int[n];
        for (int i = 0; i < n; i++) {
            arr[i]= scanner.nextInt();
        }
        solve(arr);
    }
    public static void solve(int[] arr){
        ArrayList<ArrayList<Integer>> lists=new ArrayList<>();
        ArrayList<Integer> tmp=new ArrayList<>();
        test(lists,tmp,arr,0);
        System.out.println(lists);
    }
    public static void test(ArrayList<ArrayList<Integer>> list,ArrayList<Integer> tmp,int[] arr,int index){
        if (index==arr.length){
            list.add(new ArrayList<>(tmp));
            return;
        }
        tmp.add(arr[index]);
        test(list,tmp,arr,index+1);
        tmp.remove(tmp.size()-1);
        test(list,tmp,arr,index+1);
    }
}