import java.util.ArrayList;

import java.util.List;

/**

* Description: 组合

* Date: 2020-11-20

* Time: 10:10 PM

*/

public class Combine {

public List<List<Integer>> combine(int n, int k) {

List<List<Integer>> res = new ArrayList<>();

//terminator

if (n <= 0 || n < k || k == 0) {

return res;

}

//process

//if choose the n'th num

res = combine(n - 1, k - 1);

if (res.isEmpty()) {

res.add(new ArrayList<>());

}

for (List<Integer> list : res) {

list.add(n);

}

//if not choose the n'th num

res.addAll(combine(n - 1, k));

return res;

}

public static void main(String[] args) {

System.out.println(new Combine().combine(6, 2));

}

}