using System;

​

public class Program

{

    public static void Main()

    {

        var s1 = "abc";

        var s2 = "axcc";

        var isOneAway = IsOneAway(s1, s2);

        Console.WriteLine("isOneAway :" + isOneAway);

    }

​

    public static bool IsOneAway(string s1, string s2)

    {

        if (s1 == s2)

            return false;

        if (Math.Abs(s1.Length - s2.Length) > 1)

            return false;

        if (s1 == "" && s2.Length == 1)

            return true;

        for (var i = 0; i < s1.Length; i++)

        {

            if (s2.Length > i)

            {

                if (s2[i] != s1[i])

                {

                    if (s1.Length == s2.Length)

                    {

                        return (s2.Substring(0, i) + s1[i] + s2.Substring(i + 1)) == s1;

                    }

​

                    if (s1.Length > s2.Length)

                    {

                        return (s2.Substring(0, i) + s1[i] + s2.Substring(i)) == s1;

                    }

​

                    if (s1.Length < s2.Length)

                    {

                        return (s2.Substring(0, i) + s2.Substring(i + 1)) == s1;

                    }

                }

            }

            else

            {

                return s2 + s1[i] == s1;

            }

        }

​

        return s1 == s2.Substring(0, s2.Length - 1);

    }

}