144. Binary Tree Preorder Traversal

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

For example:

Given binary tree [1,null,2,3],

1
\
2
/
3

return [1,2,3].

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

/**
* 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> list = new ArrayList<Integer>();

public List<Integer> preorderTraversal(TreeNode root) {
preOrder(root);
return list;
}

public void preOrder(TreeNode node) {
if (node == null) {
return;
}
list.add(node.val);
if (node.left != null) {
preOrder(node.left);
}
if (node.right != null) {
preOrder(node.right);
}
}
}