Binary Tree to DLL

class Solution
{
    private:
    Node* prev = NULL;
    void convertToDLL(Node* root)
    {
       if (root == NULL) return;
        convertToDLL(root->left);
        if (prev == NULL)
        {
            prev = root;
        }
        else
        {
            root->left = prev;
            prev->right = root;
            prev = root;
        }
        convertToDLL(root->right);
    }
    public: 
    Node * bToDLL(Node *root)
    {
        if (root == NULL) return NULL;
        convertToDLL(root);
        while (root->left != NULL)
        {
            root = root->left;
        }
        
        return root;
    }
};