859. Buddy Strings

class Solution:
# time O(N) space O(N)
    def buddyStrings(self, A: str, B: str) -> bool:
        #early termination: len_a != len_b
        if len(A) != len(B):
            return False
        # case1: A == B
        if A == B and len(A) > len(set(A)):
            print('here')
            return True 
        # case2: A != B, be careful there only allow to swap once 
        if A != B:
            diff = [(a,b) for a,b in zip(A,B) if a != b]
            # why need to compare each different char?   e.g. "abcaa" "abcbb"
            if len(diff) == 2 and diff[0][::-1] == diff[1]:
                return True
        return False