Given two strings a and b, return the length of the longest uncommon subsequence between a and b. If the longest uncommon subsequence does not exist, return -1.
An uncommon subsequence between two strings is a string that is a subsequence of one but not the other.
A subsequence of a string s is a string that can be obtained after deleting any number of characters from s.
For example, "abc" is a subsequence of "aebdc" because you can delete the underlined characters in "aebdc" to get "abc". Other subsequences of "aebdc" include "aebdc", "aeb", and "" (empty string).
it's easy to think about, if len(a) > len(b), we can use a as the substring, b cannot form a. In the same way, we will choose len(b) when len(b) > len(a). When len(a) == len(b), if a != b we can choose a as the substring. only when a == b, we need to return -1.
AC