145. Binary Tree Postorder Traversal - cocoder39/coco39_LC GitHub Wiki

pre:         root -> left -> right
our process: root -> right -> left (from pre to this process, we always go to right child instead of left child)
post:        left -> right -> root (from our process to post, we reverse the result)

vector<int> res;
        stack<TreeNode*> s;
        while (root || ! s.empty()) {
            if (root) {
                res.push_back(root->val);
                s.push(root);
                root = root->right;
            }
            else { //! root && ! S.empty()
                TreeNode* node = s.top();
                s.pop();
                root = node->left;
            }
        }
        reverse(res.begin(), res.end());
        return res;