854 - K-Similar Strings.

Hard

Strings s1 and s2 are k \-similar (for some non-negative integer k) if we can swap the positions of two letters in s1 exactly k times so that the resulting string equals s2.

Given two anagrams s1 and s2, return the smallest k for which s1 and s2 are k \-similar.

Example 1:

Input: s1 = "ab", s2 = "ba"

Output: 1

Explanation: The two string are 1-similar because we can use one swap to change s1 to s2: "ab" --> "ba".

Example 2:

Input: s1 = "abc", s2 = "bca"

Output: 2

Explanation: The two strings are 2-similar because we can use two swaps to change s1 to s2: "abc" --> "bac" --> "bca".

Constraints:

• 1 <= s1.length <= 20

• s2.length == s1.length

• s1 and s2 contain only lowercase letters from the set {'a', 'b', 'c', 'd', 'e', 'f'}.

• s2 is an anagram of s1.