emoji_b_tree_validation.swift

enum Node<T> where T: Equatable & Comparable {
    case empty
    indirect case value(left: Node<T>, value: T, right: Node<T>)
}

extension Node {
    func isValid(within range: ClosedRange<T>) -> Bool {
        switch self {
        case .empty:
            return true
        case let .value(left, value, right):
            guard range.contains(value) else { return false }
            return left.isValid(within: range.lowerBound...value)
                && right.isValid(within: value...range.upperBound)
        }
    }
}

let tree = Node<Int>.value(left: .value(left: .empty,
                                        value: 10,
                                        right: .empty),
                           value: 12,
                           right: .empty)

print(tree.isValid(within: Int.min...Int.max)) // => true

// Given that emoji are just unicode under the hood, 
// they are sequential and can therefore be used to 
// create a closed countable range :joy:

let emojiTree = Node<Character>.value(left: .value(left: .empty,
                                                   value: "😅",
                                                   right: .empty),
                                      value: "😎",
                                      right: .empty)

print(emojiTree.isValid(within: "😀"..."🥳"))  // => true