This problem was asked by AirBnB.
Given two strings A and B of lowercase letters, return true if and only if we can swap two letters in A so that the result equals B.
Input: A = "ab", B = "ba"
Output: True
Input: A = "ab", B = "ba"
Output: False
Input: A = "aa", B = "aa"
Output: True
Input: A = "aaaaaaabc", B = "aaaaaaacb"
Output: True
Input: A = "", B = "aa"
Output: False