class 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)
    }

     func pushCharacter(c: Character) {
        if letters.contains(c) {
            self.stack.insert(c, at: self.stack.endIndex)
        }

    }

     func enqueueCharacter(c: Character)  {
        if letters.contains(c) {
           self.queue.insert(c, at: self.queue.endIndex)
        }

    }

     func popCharacter() ->Character {
        if let lastChar = self.stack.popLast() {
            return lastChar
            }
    }

     func dequeueCharacter() ->Character {
        let firstChar = self.queue.remove(at: 0)
        return firstChar


}