Stack.swift

struct Stack<T>: CustomStringConvertible {
    
    // array of items
    var items: [T] = []
    
    // to print the formatted description
    var description: String {
        return "---- Stack begin ----\n" +
            items.map({ "\($0)" }).joined(separator: "\n") +
        "\n---- Stack End ----"
    }
    
    // push
    mutating func push(_ item: T) {
        self.items.insert(item, at: 0)
    }
    
    // pop
    @discardableResult
    mutating func pop() -> T? {
        return self.items.removeFirst()
    }
    
    // peek item
    @discardableResult
    func peek() -> T? {
        guard let firstItem = self.items.first else { return nil }
        return firstItem
    }
}