1:      /// <summary>

   2:      /// Assume you have a method isSubstring which checks if one word is a substring of another. 

   3:      /// Given two strings, s1 and s2, 

   4:      /// write code to check if s2 is a rotation of s1 using only one call to 

   5:      /// isSubstring (i.e., “waterbottle” is a rotation of “erbottlewat”).

   6:      /// </summary>

   7:      class Program

   8:      {

   9:          static void Main(string[] args)

  10:          {

  11:              string s1 = "waterbottle";

  12:              string s2 = "erbottlewat";

  13:              Program p = new Program();

  14:              bool r = p.IsARotation(s1, s2);

  15:          }

  16:   

  17:          public bool IsARotation(string s1, string s2)

  18:          {

  19:              if (string.IsNullOrEmpty(s1) || string.IsNullOrEmpty(s2))

  20:              {

  21:                  throw new ArgumentNullException("please do not input empty or null string");

  22:              }

  23:   

  24:              if (s1.Length != s2.Length)

  25:              {

  26:                  return false;

  27:              }

  28:   

  29:              string ns = s1 + s1;

  30:   

  31:              return ns.Contains(s2);

  32:          }

  33:      }