144. Binary Tree Preorder Traversal

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?

Java Code:

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List<Integer> preorderTraversal(TreeNode root) {        List<Integer> result = new ArrayList<>();        if (root == null) {            return result;        }        Stack<TreeNode> stack = new Stack<TreeNode>();        stack.push(null);        TreeNode node = root;        do {            if (node != null) {                stack.push(node);                result.add(node.val);                node = node.left;            } else {                node = stack.pop();                node = node == null ? null : node.right;            }        } while (!stack.isEmpty());                return result;    }}