struct Solution {
    private var stack = [Character]()
    private var queue = [Character]()
    private let letters = Set<Character>("qwertyuiopasdfghjklzxcvbnm".characters)
    private enum SolutionErrors: Error {
        case invalidChar(String)
        case isEmpty(String)
    }

    mutating func pushCharacter(c: Character) throws {
        guard letters.contains(c) else {
            throw SolutionErrors.invalidChar("This data structure accepts only letters")
        }
        self.stack.insert(c, at: self.stack.endIndex)
    }

    mutating func enqueueCharacter(c: Character) throws {
        guard letters.contains(c) else {
            throw SolutionErrors.invalidChar("This data structure accepts only letters")
        }
        self.queue.insert(c, at: self.queue.endIndex)
    }

    mutating func popCharacter() throws->Character {
        if self.stack.isEmpty {
            throw SolutionErrors.isEmpty("This data structure is empty")
        }
        else {
            if let lastChar = self.stack.popLast() {
            return lastChar
            }
        }
    }

    mutating func dequeueCharacter() throws->Character {
        if self.queue.isEmpty {
            throw SolutionErrors.isEmpty("This data structure is empty")
        }
        else {
            let firstChar = self.queue.remove(at: 0)
            return firstChar
        }
    }


}