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?

Solution:
A stack is necessary when there is no parent pointer available in the tree.
Pre-order: root-left-right. We are going to traverse left child first, accordingly,  we will push right child first.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList preorderTraversal(TreeNode root) {
        ArrayList ret = new ArrayList();
        Stack stack = new Stack();
        
        if(root == null) return ret;
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode node = stack.pop();
            ret.add(node.val);
            if(node.right != null) stack.push(node.right);
            if(node.left != null) stack.push(node.left);
        }
        return ret;
    }
}