# LeetCode 144. Binary Tree Preorder Traversal

## Description

https://leetcode.com/problems/binary-tree-preorder-traversal/description/

Given a binary tree, return the preorder traversal of its nodes’ values.

For example:
Given binary tree `{1,#,2,3}`,

```   1
\
2
/
3
```

return `[1,2,3]`.

Note: Recursive solution is trivial, could you do it iteratively?

## Explanation

To return the preorder traversal of tree nodes values iteratively, we introduce a stack. With a stack, we can easily iterative nodes with preorder (root, left, right) sequence.

## Java Solution

```/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();

if (root == null) {
return result;
}

stack.push(root);
while (!stack.empty()) {
TreeNode node = stack.pop();