An enum based BinaryTree implementation.

EnumBinaryTree.swift

enum BinaryTree<Element> {
    case leaf
    indirect case node(Element, l: BinaryTree<Element>, r: BinaryTree<Element>)
}

extension BinaryTree {
    init(_ value: Element) {
        self = .node(value, l: .leaf, r: .leaf)
    }
}

extension BinaryTree {
    var values: [Element] {
        switch self {
            case .leaf:
                return []
            case let .node(el, left, right):
                return left.values + [el] + right.values
        }
    }
    
    func display() -> String {
        switch self {
        case let .node(el, left, right):
            var s = ""
            s = "    \(el)\n  +  + \n \(left.display())    \(right.display()))"
            return s
        default: return ""
        }
    }
}

let tree: BinaryTree<Int> = .node(5, l: .node(4, l: .leaf, r: .node(3, l: .leaf, r: .leaf)), r: .node(2, l: .leaf, r: .leaf))
print(tree.values)
print(tree.display())