change basic logic for independent test

Author: Ashwagandha-coder

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
 }

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())
     }