class Solution
{
public:
    void inorder_dfs(Node * node, Node * & parent) {
        if(node == NULL) {
            return;
        }
        inorder_dfs(node->left, parent);
        parent->left = NULL;
        parent->right = node;
        parent = node;
        inorder_dfs(node->right, parent);
    }
    Node *flattenBST(Node *root)
    {
        Node * tmp = new Node(-1);
        Node * parent = tmp;
        inorder_dfs(root, parent);
        parent->left = NULL;
        parent->right = NULL;
        Node * res = tmp->right;
        return res;
    }
};