import java.util.ArrayList;

import java.util.List;

public class Combination {

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

int[] nums = new int[n];

for(int i = 1; i <= n; i++)

nums[i - 1] = i;

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

if(nums.length == 0 || nums.length < k)

return res;

permutation(nums, new ArrayList<Integer>(), res, k, 0);

return res;

}

private void permutation(int[] nums, List<Integer> one, List<List<Integer>> res, int k, int start){

int len = nums.length;

if(one.size() == k){

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

}else{

for(int i = start; i < len; i++){

if(one.contains(nums[i])) continue;

one.add(nums[i]);

permutation(nums, one, res, k, i + 1);

one.remove(one.size() - 1);

}

}

}

public static void main(String[] args) {

Combination combination = new Combination();

System.out.println(combination.combine(4, 2));

}

}