static string ArrayToString(string[] arr)
        {
            StringBuilder builder = new StringBuilder();
            foreach(string str in arr)
            {
                builder.Append(str);
            }

            return builder.ToString();
        }

        static string LongestMatch(string[] book1, string[] book2)
        {

            string longestMatch = "";
            string current = "";
            string text1 = ArrayToString(book1);
            string text2 = ArrayToString(book2);

            int startId = 0;

            for (int i = startId; i < text1.Length; i++)
            {
                current += text1[i];
                if(Regex.Match(text2,@current).Success && longestMatch.Length < current.Length)
                {
                    longestMatch = current;
                }
                else if(!Regex.Match(text2, @current).Success)
                {
                    current = "";
                    startId++;
                    i = startId;
                }
            }

            return longestMatch;
        }