vector<int> res;

    vector<int> postorderTraversal(TreeNode* root) {

        if (!root) return res;

        if (root->left) postorderTraversal(root->left);

        if (root->right) postorderTraversal(root->right);

        res.push_back(root->val);

        return res;

    }

    vector<int> postorderTraversal(TreeNode* root) {

        vector<int> res;

        if (!root) return res;

        stack<TreeNode*> S;

        TreeNode* p=root, *r=nullptr;

        while (p||!S.empty())

        {

            if (p)

            {

                S.push(p);

                p=p->left;

            }

            else

            {

                p=S.top();

                if (p->right&&p->right!=r)

                    p=p->right;

                else

                {

                    S.pop();

                    res.push_back(p->val);

                    r=p;

                    p=nullptr;

                }

            }

        }

        return res;

    }

    vector<int> postorderTraversal(TreeNode* root) {

        vector<int> res;

        if (!root) return res;

        stack<TreeNode*> S;

        TreeNode* p=root;

        S.push(p);

        while (!S.empty())

        {

            p=S.top();

            S.pop();

            if (p->left) S.push(p->left);

            if (p->right) S.push(p->right);

            res.insert(res.begin(),p->val);

        }

        return res;

    }