func commonChars(_ inputs: [String]) -> [String] {
    guard let lastString = inputs.last else { return [] }

    var commonCountedChars = CountedSet<String>()
    for char in lastString {
        commonCountedChars.add(String(char))
    }
    var strings = inputs
    strings.removeLast()
    // removing last is more computationally efficient than removing first in Swift

    for string in strings {
        var stringCountedChars = CountedSet<String>()
        for char in string {
            stringCountedChars.add(String(char))
        }
        for (commonChar, commonCount) in commonCountedChars.asDictionary {
            if let stringCharCount = stringCountedChars.count(for: commonChar) {
                if stringCharCount < commonCount {
                    commonCountedChars.remove(commonChar, count: commonCount - stringCharCount)
                }
            } else {
                commonCountedChars.removeAll(of: commonChar)
            }
        }
    }

    return commonCountedChars.asArray
}

struct CountedSet<Element: Hashable> {
    private var dict = [Element: UInt]()

    var asArray: [Element] {
        var array = [Element]()
        for (element, count) in dict {
            array.append(contentsOf: [Element](repeating: element, count: Int(count)))
        }
        return array
    }

    var asDictionary: [Element: UInt] { dict }

    init() {}

    init(from array: [Element]) {
        for element in array {
            if self.dict.keys.contains(element) {
                self.dict[element]! += 1
            } else {
                self.dict[element] = 1
            }
        }
    }

    func count(for element: Element) -> UInt? {
        return (dict[element] != nil) ? dict[element]! : nil
    }

    mutating func add(_ element: Element) {
        dict[element] = (dict[element] == nil) ? 1 : dict[element]! + 1
    }

    mutating func remove(_ element: Element, count removingCount: UInt = 1) {
        if let currentCount = dict[element] {
            dict[element] = (currentCount <= removingCount) ? nil : currentCount - removingCount
        }
    }

    mutating func removeAll(of element: Element) {
        dict[element] = nil
    }
}

print(commonChars(["bella","label","roller"]))
print(commonChars(["cool","lock","cook"]))