feat: stack-safe traversal + lazy Sequence + navigation/functional extensions

Core additive work for v3.4 (non-breaking):

- Rewrite nodeCount(), height(), clear() and the post-order iterator iteratively
  so deep/degenerate trees no longer throw StackOverflowError (verified to 50k deep).
- Add lazy Sequence traversal: asSequence(order), pre/post/levelOrderSequence().
- Add navigation extensions: isLeaf, degree, root(), ancestors(), siblings(),
  leaves(), descendants().
- Add functional extensions: findNode, filterNodes, anyNode, allNodes, countNodes,
  foldNodes, mapValues, deepCopy, structurallyEquals (all stack-safe).
- Add tests for stack-safety, the new APIs, and previously-uncovered
  height/depth/nodeCount/path (incl. exception paths). 40 tests green on JVM.

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/TreeNode.kt

@@ -72,20 +72,30 @@ open class TreeNode<T>(val value: T, var treeIterator: TreeNodeIterators = PreOr
      * @return All child and nested child count.
      */
     fun nodeCount(): Int {
-        if (_children.isEmpty())
-            return 0
-        return _children.size +
-                _children.sumOf { it.nodeCount() }
+        var count = 0
+        val stack = ArrayDeque<TreeNode<T>>()
+        stack.addAll(_children)
+        while (stack.isNotEmpty()) {
+            val node = stack.removeLast()
+            count++
+            stack.addAll(node._children)
+        }
+        return count
     }
 
     /**
      * @return The number of edges on the longest path between current node and a descendant leaf.
      */
     fun height(): Int {
-        val childrenMaxDepth = _children.map { it.height() }
-            .maxOrNull()
-            ?: -1 // -1 because this method counts nodes, and edges are always one less then nodes.
-        return childrenMaxDepth + 1
+        var maxDepth = 0
+        val stack = ArrayDeque<Pair<TreeNode<T>, Int>>()
+        stack.addLast(this to 0)
+        while (stack.isNotEmpty()) {
+            val (node, depthSoFar) = stack.removeLast()
+            if (depthSoFar > maxDepth) maxDepth = depthSoFar
+            node._children.forEach { stack.addLast(it to depthSoFar + 1) }
+        }
+        return maxDepth
     }
 
     /**
@@ -132,9 +142,18 @@ open class TreeNode<T>(val value: T, var treeIterator: TreeNodeIterators = PreOr
      * Remove all children from root and every node in tree.
      */
     fun clear() {
-        _parent = null
-        _children.forEach { it.clear() }
-        _children.clear()
+        val all = ArrayDeque<TreeNode<T>>()
+        val stack = ArrayDeque<TreeNode<T>>()
+        stack.addLast(this)
+        while (stack.isNotEmpty()) {
+            val node = stack.removeLast()
+            all.addLast(node)
+            stack.addAll(node._children)
+        }
+        all.forEach { node ->
+            node._parent = null
+            node._children.clear()
+        }
     }
 
     override fun toString(): String {

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/TreeNodeFunctionalExt.kt

@@ -0,0 +1,65 @@
+package com.github.adriankuta.datastructure.tree
+
+/** Returns the first node (pre-order) whose value matches [predicate], or `null`. Short-circuits. */
+fun <T> TreeNode<T>.findNode(predicate: (T) -> Boolean): TreeNode<T>? =
+    preOrderSequence().firstOrNull { predicate(it.value) }
+
+/** All nodes (pre-order) whose value matches [predicate]. */
+fun <T> TreeNode<T>.filterNodes(predicate: (T) -> Boolean): List<TreeNode<T>> =
+    preOrderSequence().filter { predicate(it.value) }.toList()
+
+/** `true` if any node's value matches [predicate]. Short-circuits. */
+fun <T> TreeNode<T>.anyNode(predicate: (T) -> Boolean): Boolean =
+    preOrderSequence().any { predicate(it.value) }
+
+/** `true` if every node's value matches [predicate]. Short-circuits. */
+fun <T> TreeNode<T>.allNodes(predicate: (T) -> Boolean): Boolean =
+    preOrderSequence().all { predicate(it.value) }
+
+/** Counts nodes whose value matches [predicate]. */
+fun <T> TreeNode<T>.countNodes(predicate: (T) -> Boolean): Int =
+    preOrderSequence().count { predicate(it.value) }
+
+/** Folds [operation] over all nodes in pre-order, starting from [initial]. */
+fun <T, R> TreeNode<T>.foldNodes(initial: R, operation: (acc: R, node: TreeNode<T>) -> R): R =
+    preOrderSequence().fold(initial) { acc, node -> operation(acc, node) }
+
+/**
+ * Returns a new tree with the same shape whose values are produced by [transform]. The original is
+ * left untouched. Stack-safe (iterative), so it handles arbitrarily deep trees.
+ */
+fun <T, R> TreeNode<T>.mapValues(transform: (T) -> R): TreeNode<R> {
+    val newRoot = TreeNode(transform(value), treeIterator)
+    val stack = ArrayDeque<Pair<TreeNode<T>, TreeNode<R>>>()
+    stack.addLast(this to newRoot)
+    while (stack.isNotEmpty()) {
+        val (source, target) = stack.removeLast()
+        source.children.forEach { child ->
+            val mappedChild = TreeNode(transform(child.value), child.treeIterator)
+            target.addChild(mappedChild)
+            stack.addLast(child to mappedChild)
+        }
+    }
+    return newRoot
+}
+
+/** Returns an independent deep copy of this subtree (same values, same shape, new nodes). */
+fun <T> TreeNode<T>.deepCopy(): TreeNode<T> = mapValues { it }
+
+/**
+ * Structural equality: `true` when [other] holds the same values in the same shape. Unlike
+ * [TreeNode]'s reference equality, this compares the entire subtree. Stack-safe.
+ */
+fun <T> TreeNode<T>.structurallyEquals(other: TreeNode<T>): Boolean {
+    val stack = ArrayDeque<Pair<TreeNode<T>, TreeNode<T>>>()
+    stack.addLast(this to other)
+    while (stack.isNotEmpty()) {
+        val (a, b) = stack.removeLast()
+        if (a.value != b.value) return false
+        if (a.children.size != b.children.size) return false
+        for (i in a.children.indices) {
+            stack.addLast(a.children[i] to b.children[i])
+        }
+    }
+    return true
+}

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/TreeNodeNavigationExt.kt

@@ -0,0 +1,41 @@
+package com.github.adriankuta.datastructure.tree
+
+/** `true` when this node has no children. */
+val <T> TreeNode<T>.isLeaf: Boolean get() = children.isEmpty()
+
+/** The number of direct children of this node. */
+val <T> TreeNode<T>.degree: Int get() = children.size
+
+/** Walks up the parent chain and returns the topmost ancestor (the tree root). */
+fun <T> TreeNode<T>.root(): TreeNode<T> {
+    var node = this
+    var parent = node.parent
+    while (parent != null) {
+        node = parent
+        parent = node.parent
+    }
+    return node
+}
+
+/** The chain of ancestors from the immediate [parent] up to (and including) the root. */
+fun <T> TreeNode<T>.ancestors(): List<TreeNode<T>> {
+    val result = mutableListOf<TreeNode<T>>()
+    var parent = this.parent
+    while (parent != null) {
+        result.add(parent)
+        parent = parent.parent
+    }
+    return result
+}
+
+/** The other children of this node's parent (excludes this node). Empty for the root. */
+fun <T> TreeNode<T>.siblings(): List<TreeNode<T>> =
+    parent?.children?.filter { it !== this } ?: emptyList()
+
+/** All leaf nodes in this subtree, in pre-order. */
+fun <T> TreeNode<T>.leaves(): List<TreeNode<T>> =
+    preOrderSequence().filter { it.isLeaf }.toList()
+
+/** All nodes in this subtree except this node, in pre-order. */
+fun <T> TreeNode<T>.descendants(): List<TreeNode<T>> =
+    preOrderSequence().filter { it !== this }.toList()

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/TreeNodeSequenceExt.kt

@@ -0,0 +1,31 @@
+package com.github.adriankuta.datastructure.tree
+
+import com.github.adriankuta.datastructure.tree.iterators.LevelOrderTreeIterator
+import com.github.adriankuta.datastructure.tree.iterators.PostOrderTreeIterator
+import com.github.adriankuta.datastructure.tree.iterators.PreOrderTreeIterator
+import com.github.adriankuta.datastructure.tree.iterators.TreeNodeIterators
+
+/**
+ * Lazily traverses this subtree in the given [order] as a [Sequence], without forcing the whole
+ * traversal up front. Pairs with the Kotlin stdlib, e.g.
+ * `root.asSequence().map { it.value }.firstOrNull { it == target }`.
+ */
+fun <T> TreeNode<T>.asSequence(
+    order: TreeNodeIterators = TreeNodeIterators.PreOrder,
+): Sequence<TreeNode<T>> {
+    val self = this
+    return when (order) {
+        TreeNodeIterators.PreOrder -> Sequence { PreOrderTreeIterator(self) }
+        TreeNodeIterators.PostOrder -> Sequence { PostOrderTreeIterator(self) }
+        TreeNodeIterators.LevelOrder -> Sequence { LevelOrderTreeIterator(self) }
+    }
+}
+
+/** Lazy pre-order traversal as a [Sequence]. */
+fun <T> TreeNode<T>.preOrderSequence(): Sequence<TreeNode<T>> = asSequence(TreeNodeIterators.PreOrder)
+
+/** Lazy post-order traversal as a [Sequence]. */
+fun <T> TreeNode<T>.postOrderSequence(): Sequence<TreeNode<T>> = asSequence(TreeNodeIterators.PostOrder)
+
+/** Lazy level-order (breadth-first) traversal as a [Sequence]. */
+fun <T> TreeNode<T>.levelOrderSequence(): Sequence<TreeNode<T>> = asSequence(TreeNodeIterators.LevelOrder)

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/iterators/PostOrderTreeIterator.kt

@@ -22,27 +22,22 @@ import com.github.adriankuta.datastructure.tree.TreeNode
  */
 class PostOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
 
-    private val stack = ArrayDeque<TreeNode<T>>()
+    private val result = ArrayDeque<TreeNode<T>>()
 
     init {
-        stack.addAll(getChildrenStack(root))
-    }
-
-    override fun hasNext(): Boolean = stack.isNotEmpty()
-
-    override fun next(): TreeNode<T> {
-        return stack.removeFirst()
-    }
-
-    private fun getChildrenStack(node: TreeNode<T>): ArrayDeque<TreeNode<T>> {
+        // Iterative post-order: pop a node, prepend it to `result`, then push its children
+        // left-to-right. Reading `result` front-to-back yields post-order — without the deep
+        // recursion that overflowed the stack on degenerate (linear) trees.
         val stack = ArrayDeque<TreeNode<T>>()
-        if(node.children.isEmpty()) {
-            return ArrayDeque(listOf(node))
+        stack.addLast(root)
+        while (stack.isNotEmpty()) {
+            val node = stack.removeLast()
+            result.addFirst(node)
+            node.children.forEach { stack.addLast(it) }
         }
-        node.children.forEach {
-            stack.addAll(getChildrenStack(it))
-        }
-        stack.addLast(node)
-        return stack
     }
+
+    override fun hasNext(): Boolean = result.isNotEmpty()
+
+    override fun next(): TreeNode<T> = result.removeFirst()
 }