UsingAnArraysAsAQueue.swift

//First-in first-out 
//concession-stand line 
//i.e. enqueue(4), enqueue(43), enqueue(3), enqueue(2) 
//-> dequeue() returns 4 
//append items and take items from [0] or .first to get next item 
//or: add items to beginning at index 0 and rettrieve items from the end 

var numbers = [1,2,3]
    
func enqueue(n: Int) {
    numbers.append(n)
}

func dequeue() -> Int? {
    guard let first = numbers.first else { return nil }
    numbers.remove(at: 0)
    return first
}
    
 //shorthand: append() and popFirst()