stack_and_queue_based_with_array.swift

struct Stack<T> {
    private var array: [T] = []
    
    var top: T? { array.last }
    
    mutating func push(_ element: T) {
        array.append(element)
    }
    
    mutating func pop() -> T? {
        return array.popLast()
    }
}

struct Queue<T> {
    private var array: [T] = []
    
    var head: T? { array.first }
    var tail: T? { array.last }
    
    mutating func enqueue(_ element: T) {
        array.append(element)
    }
    
    mutating func dequeue() -> T? {
        return array[array.indices].popFirst()
    }
}

var stack = Stack<Int>()
var queue = Queue<Int>()

stack.push(1)
stack.push(2)
stack.pop() // 2

queue.enqueue(1)
queue.enqueue(2)
queue.dequeue() // 1