Node MergeLists(Node headA, Node headB) {
    if(headA == null) return headB;
    if(headB == null) return headA;

    Node lowest;
    if(headA.data < headB.data) {
        lowest = headA;
        headA.next = MergeLists(headA.next, headB);
    } else {
        lowest = headB;
        headB.next = MergeLists(headA, headB.next);
    }

    return lowest;
}