combination的eclipse运行结果

import java.util.ArrayList;
import java.util.Arrays;


public class Combination {
    
    public static ArrayList<ArrayList<Integer>> combine(int n, int k) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        if(n <= 0||n < k)
            return res;
        ArrayList<Integer> item = new ArrayList<Integer>();    
        dfs(n,k,1,item, res);//because it need to begin from 1
        return res;
    }
    
    private static void dfs(int n, int k, int start, ArrayList<Integer> item, ArrayList<ArrayList<Integer>> res){
        if(item.size()==k){
            res.add(new ArrayList<Integer>(item));//because item is ArrayList<T> so it will not disappear from stack to stack
            System.out.println(item);
            return;
        }
        for(int i=start;i<=n;i++){
            System.out.println(i);
            item.add(i);
            dfs(n,k,i+1,item,res);
            System.out.println(i);
            item.remove(item.size()-1);
            System.out.println(item);

        }
    }
    
    public static void main(String[] args)
    {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        res=combine(3,2);
        System.out.println(res);

      }

}

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