using System;

public class Program

{

    public static void Main()

    {

        char[] arr={'a','b','c','d'};

        for(int i = 1; i < 4; i++)

        {               

            foo(arr, string.Empty, i);

        }

    }

​

    static void foo(char[] arr, string s, int len)

    {

        if (s.Length == len)

        {

            Console.WriteLine(s);

            return;

        }

        for (int i = 0; i < arr.Length; i++)

        {

            string t = s;

            t += arr[i];

            foo(arr, t, len);

        }

    }

}