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:
Your algorithm should print the following:
15
1 17 20
1 5 0 2 5 7
Hint: Consider using a
Queueincluded 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...
}