Binary Tree Preorder Traversal

Problem:

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].

Analysis:

Use stack to temporarily store the nodes. Because stack is FILO, push order is from right to left. I spent about half an hour thinking the idea. It was my first time implement the preorder traversal. 

Solution:

1:  /**  
2:   * Definition for binary tree  
3:   * public class TreeNode {  
4:   *   int val;  
5:   *   TreeNode left;  
6:   *   TreeNode right;  
7:   *   TreeNode(int x) { val = x; }  
8:   * }  
9:   */  
10:  public class Solution {  
11:    public ArrayList preorderTraversal(TreeNode root) {  
12:         ArrayList result = new ArrayList();  
13:      if(root==null)  
14:       return result;  
15:      Stack<TreeNode> nodeStack=new Stack<TreeNode>();  
16:      nodeStack.push(root);  
17:      while(!nodeStack.isEmpty())  
18:      {  
19:        TreeNode current=nodeStack.pop();  
20:        if(current.right!=null) nodeStack.push(current.right);  
21:        if(current.left!=null) nodeStack.push(current.left);  
22:        result.add(current.val);  
23:      }  
24:      return result;  
25:    }  
26:  }