RansomNoteBuilder.swift

final class RansomNoteBuilder {
    func isValidRansomNote(note: String, in magazine: String) -> Bool {
        
        var lookup = magazine.reduce(into: [Character:Int]()) { dictionary, letter in
            dictionary[letter] = (dictionary[letter] ?? 0) + 1
        }
        
        for character in note {
            guard let count = lookup[character], count > 0 else { return false }
            lookup[character] = count - 1
        }
        
        return true
    }
}

RansomNoteBuilder().isValidRansomNote(note: "aabs", in: "aaabdddees")