Level-Order Traversal

Visit a tree breadth-first with a queue.

Algorithm

The canonical tree is 4(2(1,3),6(5,7)), so this Scala DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.scala

import scala.collection.mutable.{ArrayBuffer, Queue}
class Node(val value: Int, var left: Node = null, var right: Node = null)
object Main {
  def render(node: Node): String = {
    if (node == null) "_"
    else if (node.left == null && node.right == null) node.value.toString
    else s"${node.value}(${render(node.left)},${render(node.right)})"
  }
  def sampleTree(): Node = new Node(4, new Node(2, new Node(1), new Node(3)), new Node(6, new Node(5), new Node(7)))
  def listString(values: Seq[Int]): String = values.mkString("[", ", ", "]")
  def main(args: Array[String]): Unit = { val queue = Queue.empty[Node]; queue.enqueue(sampleTree()); val output = ArrayBuffer.empty[Int]; while (queue.nonEmpty) { val node = queue.dequeue(); output += node.value; if (node.left != null) queue.enqueue(node.left); if (node.right != null) queue.enqueue(node.right) }; println(listString(output)) }
}

Complexity

Time: O(n)
Space: O(w) queue space

Implementation notes

Render tree structure explicitly instead of printing node objects.
The replay highlights the node, traversal state, queue, path, or search cursor that changes at each step.

level order Level-order traversal uses a queue to visit shallower nodes first.