Flattening a Linked List

class Solution {

public:

    Node* merge(Node* a,Node* b){

        if(a==NULL)

            return b;

        if(b==NULL)

            return a;

        Node* res;

        if(a->data<=b->data){

            res = a;

            res->bottom = merge(a->bottom,b);

        }

        else{

            res = b;

            res->bottom = merge(a,b->bottom);

        }

        return res;

    }

    Node *flatten(Node *root)

    {

        // Your code here

        if(root==NULL || root->next==NULL) return root;

        

        root = merge(root,flatten(root->next));

        return root;

    }

};