Backtracking

public List<List<Integer>> solution(List<Integer> n, int m) {
    List<List<Integer>> result = new ArrayList<>();
    helper(m, result, new ArrayList<>(), n);
    return result;
}

private void helper(int size, List<List<Integer>> result, List<Integer> current, List<Integer> other) {
    // 剪枝
    if (current.size() == size) {
        result.add(new ArrayList<>(current));
        return;
    }
    for (Integer integer : other) {
        current.add(integer);
        List<Integer> newOther = new ArrayList<>(other);
        newOther.remove(integer);
        helper(size, result, current, newOther);
        // 回溯到上一层
        current.remove(current.size() - 1);
    }
}