Binary Tree Preorder Traversal

    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<Integer> result = new ArrayList<Integer>();
        if(root == null) return result;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty()) {
            TreeNode tmp = stack.pop();
            result.add(tmp.val);
            if(tmp.right != null) stack.push(tmp.right);
            if(tmp.left != null) stack.push(tmp.left);
        }
        return result;
    }