Given two integersnandk, return all possible combinations ofknumbers out of 1 ...n.

For example,
Ifn= 4 andk= 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

tag: backtracking

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> ans = new ArrayList<>();

        dfs(n, k, 1, new ArrayList<>(), ans);

        return ans;
    }

    private void dfs(int n, int k, int index, List<Integer> path, List<List<Integer>> ans){
        if (path.size() == k){
            ans.add(new ArrayList<Integer>(path));
            return;
        }

        for (int i = index; i <= n; i++){
            path.add(i);
            dfs(n, k, i + 1, path, ans);
            path.remove(path.size() - 1);
        }
    }
}