1.8 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").

1.8 checkRotation

boolean checkRotation(String str1, String str2) {
    if (str1 == null || str2 == null || str1.length() != str2.length()) {
        return false;
    }
    
    String str = str1 + str1;
    if (str.indexOf(str2) != -1) {
        return true;
    }
    else {
        return false;
    }
}