/*

    考察基本功的一道题，迭代实现二叉树前序遍历

     */

    public List<Integer> preorderTraversal(TreeNode root) {

        List<Integer> res = new ArrayList<>();

        if (root==null)

            return res;

        Stack<TreeNode> stack = new Stack<>();

        stack.push(root);

        while (!stack.isEmpty())

        {

            TreeNode cur = stack.pop();

            res.add(cur.val);

            if (cur.right!=null)

                stack.push(cur.right);

            if (cur.left!=null)

                stack.push(cur.left);

        }

        return res;

    }