vector<int> inOrder(Node* root)
    {
         vector<int>ans;
        stack<Node *>s;
        Node *curr = root;
        while(curr != NULL || !s.empty())
        {
            while(curr != NULL)
            {
                s.push(curr);
                curr = curr -> left;
            }
            curr = s.top();
            s.pop();
            ans.push_back(curr -> data);
            curr = curr -> right;
        }
        return ans;
    }