Binary Tree and Pre-In-Post order Iteration | Swift 5.0.1

//Swift 5.0.1
/*
Task: build a Binary Tree from a given array of integer in Swift.
Use the Pre, In, and Post Order iteration to print the values.

Example of tree:
     0
   1   2
  3 4 5 6
*/

// • classes and functions here •
class Node {
  var val: Int?
  var l: Node?
  var r: Node?
}

// tree builder function
func buildTree(using a: [Int], from indx: Int) -> Node {
  if a.count > indx {
    let root = Node()
    root.val = a[indx]
    root.l = buildTree(using: a, from: indx*2+1)
    root.r = buildTree(using: a, from: indx*2+2)
    return root
  } else {
    let root = Node()
    root.val = nil
    return root
  }
}

// pre-order function
func preOrder(_ root: Node) {
  if root.val != nil {
    print(root.val!)
  }
  if root.l != nil {
    preOrder(root.l!)
  }
  if root.r != nil {
    preOrder(root.r!)
  }
}

// in-order function
func inOrder(_ root: Node) {
  if root.l != nil {
    inOrder(root.l!)
  }
  if root.val != nil {
    print(root.val!)
  }
  if root.r != nil {
    inOrder(root.r!)
  }
}

// post-order function
func postOrder(_ root: Node) {
  if root.l != nil {
    postOrder(root.l!)
  }
  if root.r != nil {
    postOrder(root.r!)
  }
  if root.val != nil {
    print(root.val!)
  }
}

// • main runs from here •
let array = [0, 1, 2, 3, 4, 5, 6]
let root = buildTree(using: array, from: 0)

print("pre-order:")
preOrder(root) // output: 0, 1, 3, 4, 2, 5, 6

print("\nin-order:")
inOrder(root) // output: 3, 1, 4, 0, 5, 2, 6

print("\npost-order:")
postOrder(root) // output: 3, 4, 1, 5, 6, 2, 0