Create 0144-binary-tree-preorder-traversal.kt

Author: a93a

kotlin/0144-binary-tree-preorder-traversal.kt

@@ -0,0 +1,37 @@
+//iterative version
+class Solution {
+    fun preorderTraversal(root: TreeNode?): List<Int> {
+        val res = ArrayList<Int>()
+        val stack = Stack<TreeNode>()
+                
+        if (root != null) stack.push(root)
+        
+        while (!stack.isEmpty()) {
+            
+            val node = stack.pop()
+            res.add(node.`val`)
+            
+            if (node.right != null) stack.push(node.right)
+            if (node.left != null) stack.push(node.left)  
+        }
+        
+        return res
+    }
+}
+
+//recursion version
+class Solution {
+    fun preorderTraversal(root: TreeNode?): List<Int> {
+        val res = ArrayList<Int>()
+        
+        fun preOrder(node: TreeNode?) {
+            node?: return
+            res.add(node.`val`)
+            preOrder(node.left)
+            preOrder(node.right)
+        }
+        
+        preOrder(root)
+        return res
+    }
+}