Add level order traversal Kotlin goody (#369)

Closes #314.

Author: willmadison

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

@@ -0,0 +1,19 @@
+package traverse_level_order
+
+public data class TreeNode(val `val`: Int, var left: TreeNode? = null, var right: TreeNode? = null)
+
+public fun TreeNode?.traverseLevelOrder(): Sequence<Collection<TreeNode>> {
+  if (this == null) {
+    return sequenceOf()
+  }
+
+  var nodesAtLevel = listOf(this)
+
+  return sequence {
+    while (nodesAtLevel.isNotEmpty()) {
+      yield(nodesAtLevel)
+
+      nodesAtLevel = nodesAtLevel.flatMap { listOf(it.left, it.right) }.filterNotNull()
+    }
+  }
+}

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

@@ -0,0 +1,32 @@
+package traverse_level_order
+
+import kotlin.test.*
+
+internal class LevelOrderTraversalTest {
+
+  @Test
+  fun itTraversesLevelOrderEmptyTree() {
+    val root: TreeNode? = null
+    assertContentEquals(
+        root.traverseLevelOrder().flatMap { it }.map { it.`val` }.toList(), emptyList<Int>())
+  }
+
+  @Test
+  fun itTraversesLevelOrderSingleNode() {
+    val root: TreeNode = TreeNode(0)
+    assertContentEquals(
+        root.traverseLevelOrder().flatMap { it }.map { it.`val` }.toList(), listOf<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().flatMap { it }.map { it.`val` }.toList(),
+        listOf<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()"
+}

workspaces/adventure-pack/src/app/__tests__/__snapshots__/equip-test.ts.snap

@@ -2496,6 +2496,32 @@ public fun lcm(a: Int, b: Int): Int = a / gcd(a, b) * b
 /////////////////////////// END ADVENTURE PACK CODE ////////////////////////////"
 `;
 
+exports[`App can equip single goody: Kotlin traverseLevelOrder() 1`] = `
+"////////////////////////// BEGIN ADVENTURE PACK CODE ///////////////////////////
+// Adventure Pack commit fake-commit-hash
+// Running at: https://example.com/
+
+public data class TreeNode(val \`val\`: Int, var left: TreeNode? = null, var right: TreeNode? = null)
+
+public fun TreeNode?.traverseLevelOrder(): Sequence<Collection<TreeNode>> {
+  if (this == null) {
+    return sequenceOf()
+  }
+
+  var nodesAtLevel = listOf(this)
+
+  return sequence {
+    while (nodesAtLevel.isNotEmpty()) {
+      yield(nodesAtLevel)
+
+      nodesAtLevel = nodesAtLevel.flatMap { listOf(it.left, it.right) }.filterNotNull()
+    }
+  }
+}
+
+/////////////////////////// END ADVENTURE PACK CODE ////////////////////////////"
+`;
+
 exports[`App can equip single goody: Python 3 UnionFind 1`] = `
 "########################## BEGIN ADVENTURE PACK CODE ###########################
 # Adventure Pack commit fake-commit-hash

workspaces/adventure-pack/src/app/__tests__/__snapshots__/render-test.ts.snap

@@ -1386,6 +1386,28 @@ import gcd_int_int.gcd
 public fun lcm(a: Int, b: Int): Int = a / gcd(a, b) * b"
 `;
 
+exports[`App can render goody: Kotlin traverseLevelOrder() 1`] = `
+"package traverse_level_order
+
+public data class TreeNode(val \`val\`: Int, var left: TreeNode? = null, var right: TreeNode? = null)
+
+public fun TreeNode?.traverseLevelOrder(): Sequence<Collection<TreeNode>> {
+  if (this == null) {
+    return sequenceOf()
+  }
+
+  var nodesAtLevel = listOf(this)
+
+  return sequence {
+    while (nodesAtLevel.isNotEmpty()) {
+      yield(nodesAtLevel)
+
+      nodesAtLevel = nodesAtLevel.flatMap { listOf(it.left, it.right) }.filterNotNull()
+    }
+  }
+}"
+`;
+
 exports[`App can render goody: Python 3 UnionFind 1`] = `
 "class UnionFind: