Check whether a string is a rotation of another string or not

The first thing, of course, would be to make sure that both the strings have same length. 

Now, suppose these strings are str1 and str2 and one of them is a rotation of the other(eq. str1 = "google" and str2 = "oglego"). Append str2 to itself(making "oglegooglego"). 

Now, if we have a function to find a substring within a string, we can use it to find str1 in modified str2. If found, then str2 is a rotation of str1.