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Data Structures & Algorithms in Swift

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11 Tree Challenges
Written by Kelvin Lau

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Challenge 1: Print a tree in level order

Print all the values in a tree in an order based on their level. Nodes in the same level should be printed on the same line. For example, consider the following tree:

15 5 2 7 5 0 1 1 17 20

Your algorithm should print the following:

15
1 17 20
1 5 0 2 5 7

Hint: Consider using a Queue included for you in the Sources folder of the starter playground.

Challenge 2: Parents and ownership

Consider the original definition of a tree node:

public class TreeNode<T> {
  public var value: T
  public var children: [TreeNode] = []

  public init(_ value: T) {
    self.value = value
  }
}

Solutions

Solution to Challenge 1

A straightforward way to print the nodes in level-order is to leverage the level-order traversal using a Queue data structure. The tricky bit is determining when a newline should occur. Here’s the solution:

func printEachLevel<T>(for tree: TreeNode<T>) {
  // 1
  var queue = Queue<TreeNode<T>>()
  var nodesLeftInCurrentLevel = 0
  queue.enqueue(tree)

  // 2
  while !queue.isEmpty {

    // 3
    nodesLeftInCurrentLevel = queue.count

    // 4
    while nodesLeftInCurrentLevel > 0 {
      guard let node = queue.dequeue() else { break }
      print("\(node.value) ", terminator: "")
      node.children.forEach { queue.enqueue($0) }
      nodesLeftInCurrentLevel -= 1
    }

    // 5
    print()
  }
}

Solution to Challenge 2

You can add a property parent to the TreeNode like so:

public class TreeNode<T> {

  public weak var parent: TreeNode?

  // etc...
}
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