We are given two strings, A and B.
A shift on A consists of taking string A and moving the leftmost character to the rightmost position. For example, if A = 'abcde', then it will be 'bcdea' after one shift on A. Return True if and only if A can become B after some number of shifts on A.
return len(A)==len(B) and B in A+A
1 判断B是不是A+A中，还需要同时判断len(A)==len(B)
