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 (e.g.,"waterbottle"is a rotation
of "erbottlewat").

1 public static boolean isRotation(String s1, String s2) {2     if(s1 == null || s2 == null)    return false;3     if(s1.length() != s2.length())    return false;4 5     String s3 = s1 + s1;6     return isSubstring(s3, s2);7 }

1.8 Is Rotation