using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void SetCombinationsWithAStack(List<char> words, int take, HashSet<string> uniques)

    {

        Stack<char> stack = [];

        stack.Push(words[0]);

        int i = 0;

        List<char> visited = [], remaining;

        char word;

        while (stack.Count != 0)

        {

            word = stack.Pop();

            visited.Add(word);

            if (visited.Count >= take)

            {

                uniques.Add(string.Join(' ', visited.Order()));

                visited.Remove(word);

            }

            remaining = [..words.Where((_, index) => index != i)];

            remaining.ForEach(remainingItem => stack.Push(remainingItem));

        }

    }

​

    public static void SetCombinations(List<char> words, int take, List<char> visited, HashSet<string> uniques)

    {

        if (visited.Count >= take)

        {

            uniques.Add(string.Join(' ', visited.Order()));

            return;

        }

​

        List<char> remaining;

        for (int i = 0, l = words.Count; i < l; i++)

        {

            visited.Add(words[i]);

            remaining = [..words.Where((_, index) => index != i)];

            SetCombinations(remaining, take, visited, uniques);

            visited.Remove(words[i]);

        }

    }

​

    public static void TestCombinations(string word, int take)

    {

        var uniques = new HashSet<string>();

        SetCombinations(word.ToList(), take, [], uniques);

        Console.Write(string.Join('\n', uniques));

    }

​

    public static void TestCombinationsWithAStack(string word, int take)

    {

        var uniques = new HashSet<string>();

        SetCombinationsWithAStack(word.ToList(), take, uniques);

        Console.Write(string.Join('\n', uniques));

    }

​

    public static void Main()

    {

        var word = "abcde";

        var take = 4;

        TestCombinations(word, take);

        Console.WriteLine();

        TestCombinationsWithAStack(word, take);

    }

}