Given two stringsAandB of lowercase letters, returntrueif and only if we can swap two letters inAso that the result equalsB.

Example 1:

Input: 
A = 
"ab"
, B = 
"ba"
Output: 
true

Example 2:

Input: 
A = 
"ab"
, B = 
"ab"
Output: 
false

Example 3:

Input: 
A = 
"aa"
, B = 
"aa"
Output: 
true

Example 4:

Input: 
A = 
"aaaaaaabc"
, B = 
"aaaaaaacb"
Output: 
true

Example 5:

Input: 
A = 
""
, B = 
"aa"
Output: 
false

Note:

1. 0 < = A.length < = 20000
2. 0 < = B.length < = 20000
3. A and B consist only of lowercase letters.

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));
    }
}