use isSubstring to check if one word is a rotation of another.

    /// <summary>
    /// Assume you have a method isSubstring which checks if one word is a substring of another.
    /// Given two strings, s1 and s2,
    /// write code to check if s2 is a rotation of s1 using only one call to
    /// isSubstring (i.e., “waterbottle” is a rotation of “erbottlewat”).
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
            string s1 = "waterbottle";
            string s2 = "erbottlewat";
            Program p = new Program();
            bool r = p.IsARotation(s1, s2);
        }

        public bool IsARotation(string s1, string s2)
        {
            if (string.IsNullOrEmpty(s1) || string.IsNullOrEmpty(s2))
            {
                throw new ArgumentNullException("please do not input empty or null string");
            }

            if (s1.Length != s2.Length)
            {
                return false;
            }

            string ns = s1 + s1;

            return ns.Contains(s2);
        }
    }