Binary Tree to DLL (GFG)

class Solution
{   
    
    public: 
    Node *head = nullptr;
    void inorder(Node *root) {
        if(root == nullptr) return;
        static Node *prev = nullptr;
        inorder(root -> left);
        if(head == nullptr) {
            head = root;
        }
        else {
            root -> left = prev;
            prev -> right = root;
        }
        prev = root;
        inorder(root -> right);
    }
    
    //Function to convert binary tree to doubly linked list and return it.
    Node * bToDLL(Node *root)
    {
        // your code here
        inorder(root);
        return head;
    }
};