class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }

        Stack<TreeNode> stack = new Stack<TreeNode>();
        while (root != null || !stack.isEmpty()) {
            while (root != null ) {
                stack.push(root);
                root = root.left;
            }
            // 访问节点
            root = stack.pop();
            res.add(root.val);
            root = root.right;
        }
        return res;
    }
}

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        postorder(root, res);
        return res;
    }

    public void postorder(TreeNode root, List<Integer> res) {
        if (root == null) {
            return;
        }
        postorder(root.left, res);
        postorder(root.right, res);
        res.add(root.val);
    }
}