add the preorder traverse of binary tree

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)
+}

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
+}

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())
+    }
+}