merge sort for linked lists

struct Node
    int val;
    Node* next;
    Node(int x):val(x),next(NULL){}
};

void splitList(Node* head, Node** front, Node** back){
    Node *fast, *slow;
    slow = head;
    fast = head->next;
    
    while(fast != NULL){
        fast = fast->next;
        if(fast != NULL){
            fast = fast->next;
            slow = slow->next;
        }
    }
    *front = head;
    *back = slow->next;
    slow->next = NULL;
}

Node* sortedMerge(Node* a, Node* b){
    if(a == NULL)
        return b;
    if(b == NULL)
        return a;
    Node* mergedNode;
    if(a->val <= b->val){
        mergedNode = new Node(a->val);
        mergedNode->next = sortedMerge(a->next, b);
    }else {
        mergedNode = new Node(b->val);
        mergedNode->next = sortedMerge(a, b->next);
    }
    return mergedNode;
}

void mergeSort(Node** headRef){
    Node* head = *headRef;
    Node* a;
    Node* b;
    
    if(head == NULL || head->next == NULL)
        return;
    
    splitList(head, &a, &b);
    mergeSort(&a);
    mergeSort(&b);
    
    *headRef = sortedMerge(a, b);
}