MergeSort in linkedlist

class Solution{
  public:
    //Function to sort the given linked list using Merge Sort.
    
    Node* findMid(Node *head){
        Node* slow=head;
        Node* fast=head->next;
        while(fast && fast->next){
            slow=slow->next;
            fast=fast->next;
        }
        return slow;
    }
    
    Node* merge(Node* left,Node* right){
        if(left==NULL)
        return right;
        if(right==NULL)
        return left;
        
        Node *ans=new Node(-1);
        Node* temp=ans;
        
        while(left!=NULL && right!=NULL){
            if(left->data < right->data){
                temp->next=left;
                temp=left;
                left=left->next;
            }
            else{
                temp->next=right;
                temp=right;
                right=right->next;
            }
        }
        while(left!=NULL){
            temp->next=left;
            temp=left;
            left=left->next;
        }
        while(right!=NULL){
            temp->next=right;
            temp=right;
            right=right->next;
        }
        ans=ans->next;
        return ans;
        
    }
    Node* mergeSort(Node* head) {
        // your code here
        if(head==NULL || head->next== NULL)
        return head;
        
        Node* mid=findMid(head);
        
        Node* left=head;
        Node* right=mid->next;
        mid->next=NULL;
        
        left=mergeSort(left);
        right=mergeSort(right);
        
        Node *result=merge(left,right);
        
        return result;
    }