Merge-Sort Linked list

 class node{
        public:
            int data;
            node * next;
            node(int data){
                this->data=data;
                this->next=NULL;
            }
    };


node* findMid(node* head) {
    node* slow = head;
    node* fast = head -> next;
    
    while(fast != NULL && fast -> next != NULL) {
        slow = slow -> next;
        fast = fast -> next -> 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;
    
    //merge 2 sorted Linked List
    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) {
    //base case
    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;
}