cc150_1.8

Assume you have a method isSubstring which checks if one word is a substring
of another. Given two strings, si and s2, write code to check Ifs2 is a rotation of si
using only onecalltoisSubstring (e.g., "waterbottLe" is a rotation of "erbottLewat").

public boolean isRotation(String si, String s2){
  if(s1.length() != s2.length() || s1.length() == 0) return false;
  return isSubstring(s1+s1, s2) ;
}