Rotation of String

Problem:

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”).

这个问题的trick就是将两个原字符串拼起来，然后检测。。。