class Solution {
    public boolean buddyStrings(String A, String B) {
        if(A.length() != B.length()) return false;
                
        if(A.equals(B)){
            Set<Character> set = new HashSet<>();
            for(char c:A.toCharArray()) set.add(c);
            return set.size() < A.length();
        }
        
        List<Integer> diff = new ArrayList<>();
        for(int i=0; i<A.length(); i++)
            if(A.charAt(i) != B.charAt(i))
                diff.add(i);
        
        return diff.size()<=2 && 
            A.charAt(diff.get(0)) == B.charAt(diff.get(1)) && 
            A.charAt(diff.get(1)) == B.charAt(diff.get(0));
    }
}