Tree traversals - inOrder, preOrder, postOrder, levelOrder(Breadth first)

TreeTraversals.swift

import UIKit
import PlaygroundSupport

class TreeNode {
  var data: Int
  var left: TreeNode?
  var right: TreeNode?
  
  init(data: Int) {
    self.data = data
  }
}

class Tree {
  var root: TreeNode
  
  init(root: TreeNode) {
    self.root = root
  }
  
  ///DFS
  ///left,  root, right
  func inorderTraversal() -> [Int] {
    var result = [Int]()
    if let left = root.left {
      result.append(contentsOf: Tree(root: left).inorderTraversal())
    }
    result.append(root.data)
    if let right = root.right {
      result.append(contentsOf: Tree(root: right).inorderTraversal())
    }
    return result
  }
  
  ///DFS
  ///root, left, right
  func preOrderTraversal() -> [Int] {
    var result = [Int]()
    result.append(root.data)
    if let left = root.left {
      result.append(contentsOf: Tree(root: left).preOrderTraversal())
    }
    if let right = root.right {
      result.append(contentsOf: Tree(root: right).preOrderTraversal())
    }
    return result
  }
  
  ///DFS
  ///left,  right, root
  func postOrderTraversal() -> [Int] {
    var result = [Int]()
    if let left = root.left {
      result.append(contentsOf: Tree(root: left).postOrderTraversal())
    }
    if let right = root.right {
      result.append(contentsOf: Tree(root: right).postOrderTraversal())
    }
    result.append(root.data)
    return result
  }
  
  /// breadth first traversal
  func levelOrderTraversal() -> [Int] {
    var result = [Int]()
    var queue = [TreeNode?]()
    queue.append(root)
    while !queue.isEmpty {
      guard let node = queue.removeFirst() else { continue }
      result.append(node.data)
      queue.append(node.left)
      queue.append(node.right)
    }
    return result
  }
}

let node1 = TreeNode(data: 1)
let node2 = TreeNode(data: 2)
let node3 = TreeNode(data: 3)
let node4 = TreeNode(data: 4)
let node5 = TreeNode(data: 5)

node1.left = node2
node1.right = node3
node2.left = node4
node2.right = node5

print("inorder: ",Tree(root: node1).inorderTraversal())
print("preOrder: ",Tree(root: node1).preOrderTraversal())
print("postOrder: ",Tree(root: node1).postOrderTraversal())
print("levelOrder: ", Tree(root: node1).levelOrderTraversal())