22. BT to DLL(Recursive)

// This function should return head to the DLL
class Solution
{
    public: 
    //Function to convert binary tree to doubly linked list and return it.
    Node* solve(Node* root,Node** head,Node** tail){
        if(!root) return NULL;
        
        solve(root->left,head,tail);
        
        if(*head==NULL)
        {*head=root;
        *tail=root;}
        else{
         (*tail)->right=root;
         (*tail)->right->left=*tail;
         *tail=(*tail)->right;
        }
        
        solve(root->right,head,tail);    
        
        return root;
    }
    Node * bToDLL(Node *root)
    {
        if(!root) return NULL;
        
        Node *head=NULL;
        Node *tail=NULL;
        
        solve(root,&head,&tail);
        
        return head;
    }
};