Tree Example on iOS, written in Swift..

TreeNode.swift

import UIKit

public class TreeNode<T> {
    
    public var value: T
    public var children: [TreeNode] = []
    
    public init(_ value: T) {
        self.value = value
    }
    
    public func add(_ child: TreeNode) {
        children.append(child)
    }
    
}

extension TreeNode {
    
    public func forEachDepthFirst(visit: (TreeNode) -> Void) {
        visit(self)
        children.forEach {
            $0.forEachDepthFirst(visit: visit)
        }
    }
    
}

extension TreeNode {
    
    public func forEachLevelOrder(visit: (TreeNode) -> Void) {
        visit(self)
        var queue = QueueArray<TreeNode>()
        children.forEach { queue.enqueue($0) }
        while let node = queue.dequeue() {
            visit(node)
            node.children.forEach { queue.enqueue($0) }
        }
    }
    
}

extension TreeNode where T: Equatable {

    public func search(_ value: T) -> TreeNode? {
        var result: TreeNode?
        forEachLevelOrder { node in
            if node.value == value {
                result = node
            }
        }
        return result
    }

}


//let beverages = TreeNode("Beverages")
//let hot = TreeNode("Hot")
//let cold = TreeNode("Cold")
//
//beverages.add(hot)
//beverages.add(cold)
//beverages.children


//let depthFirstTree = makeBeverageTree()
//depthFirstTree.forEachDepthFirst { print($0.value) }
//
//
//let levelOrderTree = makeBeverageTree()
//levelOrderTree.forEachLevelOrder { print($0.value) }
//
//
//let searchTree = makeBeverageTree()
//if let searchResult1 = searchTree.search("ginger ale") {
//    print("Found node: \(searchResult1.value)")
//}
//if let searchResult2 = searchTree.search("WKD Blue") {
//    print(searchResult2.value)
//} else {
//    print("Couldn't find WKD Blue")
//}


func makeBeverageTree() -> TreeNode<String> {
    let tree = TreeNode("Beverages")
    
    let hot = TreeNode("hot")
    let cold = TreeNode("cold")
    
    let tea = TreeNode("tea")
    let coffee = TreeNode("coffee")
    let chocolate = TreeNode("cocoa")
    
    let blackTea = TreeNode("black")
    let greenTea = TreeNode("green")
    let chaiTea = TreeNode("chai")
    
    let soda = TreeNode("soda")
    let milk = TreeNode("milk")
    
    let gingerAle = TreeNode("ginger ale")
    let bitterLemon = TreeNode("bitter lemon")
    
    tree.add(hot)
    tree.add(cold)
    
    hot.add(tea)
    hot.add(coffee)
    hot.add(chocolate)
    
    cold.add(soda)
    cold.add(milk)
    
    tea.add(blackTea)
    tea.add(greenTea)
    tea.add(chaiTea)
    
    soda.add(gingerAle)
    soda.add(bitterLemon)
    
    return tree
}


public protocol Queue {
    associatedtype Element
    mutating func enqueue(_ element: Element)
    mutating func dequeue() -> Element?
    var isEmpty: Bool { get }
    var peek: Element? { get }
}

public struct QueueArray<T>: Queue {
    
    private var array: [T] = []
    
    public init() {}
    
    public var isEmpty: Bool {
        array.isEmpty
    }
    
    public var peek: T? {
        array.first
    }
    
    public mutating func enqueue(_ element: T) {
        array.append(element)
    }
    
    public mutating func dequeue() -> T? {
        isEmpty ? nil : array.removeFirst()
    }
    
}