import java.util.ArrayList;

import java.util.List;

import java.util.Stack;

public class Combine {

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

private void findCombinations(int n, int k, int begin, Stack<Integer> pre) {

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

res.add(pre);

return;

}

for (int i = begin; i <= n; i++) {

pre.push(i);

findCombinations(n, k, i + 1, pre);

pre.pop();

}

}

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

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

return res;

}

findCombinations(n, k, 1, new Stack<>());

return res;

}

}