making Yaml and BFS of QNDTree work

src/net/torvald/terrarum/QNDTreeNode.kt

@@ -1,6 +1,7 @@
 package net.torvald.terrarum
 
-import java.util.ArrayList
+import java.util.*
+import kotlin.collections.ArrayList
 
 class QNDTreeNode<T>(var data: T? = null, var parent: QNDTreeNode<T>? = null) {
     var children = ArrayList<QNDTreeNode<T>>()
@@ -18,11 +19,54 @@ class QNDTreeNode<T>(var data: T? = null, var parent: QNDTreeNode<T>? = null) {
         action(node, depth)
     }
 
+    /**
+     * (QNDTreeNode, Int) is Node-depth pair, starting from zero.
+     */
     fun traversePreorder(action: (QNDTreeNode<T>, Int) -> Unit) {
         this.traverse1(this, action)
     }
 
+    /**
+     * (QNDTreeNode, Int) is Node-depth pair, starting from zero.
+     */
     fun traversePostorder(action: (QNDTreeNode<T>, Int) -> Unit) {
         this.traverse2(this, action)
     }
+
+    /**
+     * (QNDTreeNode, Int) is Node-depth pair, starting from zero.
+     */
+    fun traverseLevelorder(action: (QNDTreeNode<T>, Int) -> Unit) {
+        val q = ArrayList<Pair<QNDTreeNode<T>, Int>>() // node, depth
+        q.add(this to 0)
+        while (q.isNotEmpty()) {
+            val node = q.removeAt(0)
+            action(node.first, node.second)
+            node.first.children.forEach {
+                q.add(it to node.second + 1)
+            }
+        }
+    }
+
+    /**
+     * Retrieves data in the node in a specific depth (level).
+     * Probably only useful for level = 1
+     */
+    fun getLevelData(level: Int): List<T?> {
+        val list = ArrayList<T?>()
+
+        traversePreorder { node, i ->
+            if (i == level) {
+                list.add(node.data)
+            }
+        }
+
+        return list
+    }
+
+    override fun toString() = data.toString()
+
+    fun print() {
+        TODO()
+    }
 }