Playing around with Tree's in Swift.

Tree.swift

protocol Show {
    func show() -> String
}

class Tree<T> : Show {
    func show() -> String { return "" }
}

class Leaf<T> : Tree<T> {
    override func show() -> String {
        return "()"
    }
}

class Node<T> : Tree<T> {
    var left_: Tree<T>
    var right_: Tree<T>
    var value: T

    init(v: T, l: Tree<T>, r: Tree<T>){
        left_  = l
        right_ = r
        value  = v
    }

    override func show() -> String {
        return "{\(self.value) \(self.left_.show()) \(self.right_.show())}"
    }
}


var one = Node<Int>(v:1, l:Leaf<Int>(), r:Leaf<Int>())
var two = Node<Int>(v:2, l:Leaf<Int>(), r:Leaf<Int>())

var root = Node<Int>(v:3, l:one, r:two)

func printTree<T>(_ t:Tree<T>) -> String {
    if let node = t as? Node<T> {
        var res = "{"
        res += String(describing:node.value)
        res += " "
        res += printTree(node.left_)
        res += " "
        res += printTree(node.right_)
        res += "}"
        return res
//        return "{" + node.value + " " + printTree(node.left_) + " " + printTree(node.right_) + "}"
    }
    return "()"
}

print(printTree(root))
print(root.show())