Add level order traversal Kotlin goody

workspaces/adventure-pack/goodies/kotlin/src/common/TreeNode.kt

@@ -0,0 +1,3 @@
+package common
+
+public data class TreeNode(val value: Int, var left: TreeNode? = null, var right: TreeNode? = null)

workspaces/adventure-pack/goodies/kotlin/src/traverse_level_order/Main.kt

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+package traverse_level_order
+
+import common.TreeNode
+
+public fun TreeNode?.traverseLevelOrder(): Sequence<TreeNode> {
+  if (this == null) return sequenceOf()
+
+  val q = ArrayDeque<TreeNode>()
+  q.add(this)
+
+  return sequence {
+    while (q.isNotEmpty()) {
+      val current = q.removeFirst()
+
+      yield(current)
+
+      if (current.left != null) {
+        q.add(current.left!!)
+      }
+
+      if (current.right != null) {
+        q.add(current.right!!)
+      }
+    }
+  }
+}

workspaces/adventure-pack/goodies/kotlin/src/traverse_level_order/Test.kt

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+package traverse_level_order
+
+import common.TreeNode
+import kotlin.test.*
+
+internal class LevelOrderTraversalTest {
+
+  @Test
+  fun itTraversesLevelOrderEmptyTree() {
+    val root: TreeNode? = null
+    assertContentEquals(root.traverseLevelOrder().map { it.value }.toList(), mutableListOf<Int>())
+  }
+
+  @Test
+  fun itTraversesLevelOrderSingleNode() {
+    val root: TreeNode = TreeNode(0)
+    assertContentEquals(root.traverseLevelOrder().map { it.value }.toList(), mutableListOf<Int>(0))
+  }
+
+  @Test
+  fun itTraversesLevelOrderFullTree() {
+    val root: TreeNode =
+        TreeNode(
+            1,
+            TreeNode(2, TreeNode(4, TreeNode(8)), TreeNode(5)),
+            TreeNode(3, TreeNode(6), TreeNode(7)))
+    assertContentEquals(
+        root.traverseLevelOrder().map { it.value }.toList(),
+        mutableListOf<Int>(1, 2, 3, 4, 5, 6, 7, 8))
+  }
+}

workspaces/adventure-pack/goodies/kotlin/src/traverse_level_order/goody.json

@@ -0,0 +1,3 @@
+{
+  "name": "traverseLevelOrder()"
+}