using System;

using System.Linq;

​

public class Program

{

    static public void Main ()

    {

        Permute("", "abcd");

    }

    static void Permute(string left, string right)

    {

        if(right.Length == 0)

        {

            Console.WriteLine(left);

            return;

        }

        foreach(char c in right)

        {

            left = left + c.ToString();

            right = new string(right.ToCharArray().Except(left.ToCharArray()).ToArray());

            Console.WriteLine(left);

            Console.WriteLine(right);

            Permute(left, right);

        }

    }

}

​