From 78e8ea04a6496e231ff81733893e760294aed390 Mon Sep 17 00:00:00 2001
From: Danil M <max.cacz@yandex.ru>
Date: Sun, 9 Nov 2025 09:06:04 -0500
Subject: [PATCH 1/3] added the preorder traverse of binary tree, and tests

---
 .../kotlin/binaryTree/PreorderTraverse.kt     | 16 ++++++++++
 src/main/kotlin/binaryTree/TreeNode.kt        |  6 ++++
 .../kotlin/binaryTree/PreorderTraverseTest.kt | 32 +++++++++++++++++++
 3 files changed, 54 insertions(+)
 create mode 100644 src/main/kotlin/binaryTree/PreorderTraverse.kt
 create mode 100644 src/main/kotlin/binaryTree/TreeNode.kt
 create mode 100644 src/test/kotlin/binaryTree/PreorderTraverseTest.kt

diff --git a/src/main/kotlin/binaryTree/PreorderTraverse.kt b/src/main/kotlin/binaryTree/PreorderTraverse.kt
new file mode 100644
index 0000000..8d01938
--- /dev/null
+++ b/src/main/kotlin/binaryTree/PreorderTraverse.kt
@@ -0,0 +1,16 @@
+package binaryTree
+
+/**
+ * This is the preorder traverse of binary tree
+ * @param root - root of TreeNode
+ * @see TreeNode - node has reference on left and right child
+ */
+
+
+fun preorderTraverse(root: TreeNode?, traverse: MutableList<Int>) {
+    root ?: return
+
+    traverse.add(root.value)
+    preorderTraverse(root.left, traverse)
+    preorderTraverse(root.right, traverse)
+}
\ No newline at end of file
diff --git a/src/main/kotlin/binaryTree/TreeNode.kt b/src/main/kotlin/binaryTree/TreeNode.kt
new file mode 100644
index 0000000..9cefde9
--- /dev/null
+++ b/src/main/kotlin/binaryTree/TreeNode.kt
@@ -0,0 +1,6 @@
+package binaryTree
+
+class TreeNode(val value: Int) {
+    var left: TreeNode? = null
+    var right: TreeNode? = null
+}
\ No newline at end of file
diff --git a/src/test/kotlin/binaryTree/PreorderTraverseTest.kt b/src/test/kotlin/binaryTree/PreorderTraverseTest.kt
new file mode 100644
index 0000000..74398ce
--- /dev/null
+++ b/src/test/kotlin/binaryTree/PreorderTraverseTest.kt
@@ -0,0 +1,32 @@
+package binaryTree
+
+import org.junit.Assert.assertArrayEquals
+import org.junit.Test
+
+class PreorderTraverseTest {
+
+    @Test
+    fun `base_case`() {
+        val root = TreeNode(1)
+        root.left = TreeNode(2)
+        root.right = TreeNode(3)
+
+        val expected = listOf(1, 2, 3)
+        val res = mutableListOf<Int>()
+        preorderTraverse(root, res)
+
+        assertArrayEquals(expected.toIntArray(), res.toIntArray())
+    }
+
+
+    @Test
+    fun `empty_tree`() {
+        val root: TreeNode? = null
+
+        val expected = emptyList<Int>()
+        val res = mutableListOf<Int>()
+        preorderTraverse(root, res)
+
+        assertArrayEquals(expected.toIntArray(), res.toIntArray())
+    }
+}
\ No newline at end of file

From d414599386abb79b1f7af076f27967942077c7bb Mon Sep 17 00:00:00 2001
From: Danil M <max.cacz@yandex.ru>
Date: Mon, 10 Nov 2025 02:37:22 -0500
Subject: [PATCH 2/3] change basic logic for independent test

---
 src/main/kotlin/binaryTree/PreorderTraverse.kt | 18 +++++++++++++-----
 .../kotlin/binaryTree/PreorderTraverseTest.kt  |  6 ++----
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/src/main/kotlin/binaryTree/PreorderTraverse.kt b/src/main/kotlin/binaryTree/PreorderTraverse.kt
index 8d01938..57bc9e6 100644
--- a/src/main/kotlin/binaryTree/PreorderTraverse.kt
+++ b/src/main/kotlin/binaryTree/PreorderTraverse.kt
@@ -7,10 +7,18 @@ package binaryTree
  */
 
 
-fun preorderTraverse(root: TreeNode?, traverse: MutableList<Int>) {
-    root ?: return
+fun preorderTraverse(root: TreeNode?): List<Int> {
+    root ?: return listOf()
+    val nodes = mutableListOf<Int>()
 
-    traverse.add(root.value)
-    preorderTraverse(root.left, traverse)
-    preorderTraverse(root.right, traverse)
+    fun dfs(root: TreeNode?) {
+        root ?: return
+
+        nodes.add(root.value)
+        dfs(root.left)
+        dfs(root.right)
+    }
+    dfs(root)
+
+    return nodes
 }
\ No newline at end of file
diff --git a/src/test/kotlin/binaryTree/PreorderTraverseTest.kt b/src/test/kotlin/binaryTree/PreorderTraverseTest.kt
index 74398ce..d37f3c9 100644
--- a/src/test/kotlin/binaryTree/PreorderTraverseTest.kt
+++ b/src/test/kotlin/binaryTree/PreorderTraverseTest.kt
@@ -12,8 +12,7 @@ class PreorderTraverseTest {
         root.right = TreeNode(3)
 
         val expected = listOf(1, 2, 3)
-        val res = mutableListOf<Int>()
-        preorderTraverse(root, res)
+        val res = preorderTraverse(root)
 
         assertArrayEquals(expected.toIntArray(), res.toIntArray())
     }
@@ -24,8 +23,7 @@ class PreorderTraverseTest {
         val root: TreeNode? = null
 
         val expected = emptyList<Int>()
-        val res = mutableListOf<Int>()
-        preorderTraverse(root, res)
+        val res = preorderTraverse(root)
 
         assertArrayEquals(expected.toIntArray(), res.toIntArray())
     }

From ff5092fc739b222cd516566c6ebc7bde5d28e345 Mon Sep 17 00:00:00 2001
From: Danial M <58209188+Ashwagandha-coder@users.noreply.github.com>
Date: Thu, 20 Nov 2025 07:01:12 -0500
Subject: [PATCH 3/3] Update src/main/kotlin/binaryTree/PreorderTraverse.kt

the small optimization for allocation list

Co-authored-by: Rita de Cassia <69485548+Rfontt@users.noreply.github.com>
---
 src/main/kotlin/binaryTree/PreorderTraverse.kt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/main/kotlin/binaryTree/PreorderTraverse.kt b/src/main/kotlin/binaryTree/PreorderTraverse.kt
index 57bc9e6..01c53a9 100644
--- a/src/main/kotlin/binaryTree/PreorderTraverse.kt
+++ b/src/main/kotlin/binaryTree/PreorderTraverse.kt
@@ -8,7 +8,7 @@ package binaryTree
 
 
 fun preorderTraverse(root: TreeNode?): List<Int> {
-    root ?: return listOf()
+    root ?: return emptyList()
     val nodes = mutableListOf<Int>()
 
     fun dfs(root: TreeNode?) {