stack.swift

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

var stack = Stack()
stack.push(element: 1)
stack.push(element: 2)
stack.push(element: 3)

stack.pop() == 3
stack.pop() == 2
stack.pop() == 1
stack.pop() == nil