stack

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while (cur != null || !stack.isEmpty())
        {
            while (cur != null)
            {
                stack.push(cur);
                cur = cur.left;
            }
            cur = stack.pop();
            list.add(cur.val);
            cur = cur.right;
        }
        return list;
    }

    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        traver(root, res, 0);
        return res;
    }
    public void traver(TreeNode root, List<List<Integer>> res, int level)
    {
        if (root == null) return;
        if (res.size() <= level)
        {
            res.add(new LinkedList<>());
        }
        List<Integer> list = res.get(level);
        if (level % 2 == 0) list.add(root.val);
        else list.add(0, root.val);
        traver(root.left, res, level + 1);
        traver(root.right, res, level + 1);
    }

    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode node = root;
        while (node != null || !stack.isEmpty())
        {
            while (node != null)
            {
                res.add(node.val);
                stack.push(node);
                node = node.left;
            }
            if (!stack.isEmpty())
            {
                node = stack.pop();
                node = node.right;
            }
        }
        return res;
    }

    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;
        while (root != null || !stack.isEmpty())
        {
            while (root != null)
            {
                stack.push(root);
                root = root.left;
            }
            if (!stack.isEmpty())
            {
                TreeNode peek = stack.peek();
                if (peek.right != null && pre != peek.right)
                {
                    root = peek.right;
                }
                else
                {
                    stack.pop();
                    res.add(peek.val);
                    pre = peek;
                }
            }
        }
        return res;
    }