Given two strings a and b, return true if you can swap two letters in a so the result is equal to b, otherwise, return false. Swapping letters is defined as taking two indices i and j (0-indexed) such that i != j and swapping the characters at a[i] and b[j]. For example, swapping at indices 0 and 2 in "abcd" results in "cbad". Example 1: Input: a = "ab", b = "ba" Output: true Explanation: You can swap a[0] = 'a' and a[1] = 'b' to get "ba", which is equal to b. Example 2: Input: a = "ab", b = "ab" Output: false Explanation: The only letters you can swap are a[0] = 'a' and a[1] = 'b', which results in "ba" != b. Example 3: Input: a = "aa", b = "aa" Output: true Explanation: You can swap a[0] = 'a' and a[1] = 'a' to get "aa", which is equal to b. Example 4: Input: a = "aaaaaaabc", b = "aaaaaaacb" Output: true Constraints: 1 <= a.length, b.length <= 2 * 104 a and b consist of lowercase letters.