[string] determine whether S2 is the rotation string 2 of S1

subject
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”).

Suppose you have a isSubstring function , You can detect whether a string is a substring of another string . Give the string s1 and s2, Use only once isSubstring Can judge s2 Whether it is s1 Rotation string for , Please write the code .
for example ：”waterbottle” yes ”erbottlewat” Rotation string for .

Ideas ：

It only needs get One point ：s1+s1 It contains s1 All rotation strings of , So it is equivalent to judgment s2 Whether it is s1+s1 The string of .

// Judge s2 Is it right? s1 The string of 
bool isSubstring(string s1, string s2){
    
    if(s1.find(s2) != string::npos) return true;
    else return false;
}
 
// Take precautions , And call :isSubstring(s1+s1, s2)
bool isRotation(string s1, string s2){
    
    if(s1.length() != s2.length() || s1.length()<=0)
        return false;
    return isSubstring(s1+s1, s2);
}

such as ：

s1 = waterbottle, A rotation string is erbottlewat

s1 + s1 = waterbottlewaterbottle