[Swift] Queue 구현하기

struct Queue<T> {
 
    private var list = [T]()
 
    var isEmpty: Bool {
 
        return self.list.isEmpty
 
    }
 
    var front: T? {
 
        return self.list.first
 
    }
 
    mutating func enqueue(_ item: T) {
 
        self.list.append(item)
 
    }
 
    mutating func dequeue() -> T? {
 
        guard self.isEmpty == false else {
 
            return nil
 
        }
 
        return self.list.removeFirst()
 
    }
 
}