Example for using the logical operators

BuddyStrings.swift

func buddyStrings(_ A: String, _ B: String) -> Bool {
    if A.count != B.count { return false }

    var A = Array(A), B = Array(B)

    if A == B {
        return A.count > Set(A).count 
    } else {
        return differentLetters(A, B)
    }
}

func differentLetters(_ A: [Character], _ B: [Character]) -> Bool {
    var indices = [Int]()
    for index in A.indices where A[index] != B[index] {
        indices.append(index)
        if indices.count > 2 { return false }
    }
    return (A[indices[0]] == B[indices[1]]) && (A[indices[1]] == B[indices[0]])
}