static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
    SinglyLinkedListNode merged = new SinglyLinkedListNode(0);
    SinglyLinkedListNode h1 = head1;
    SinglyLinkedListNode h2 = head2;
    SinglyLinkedListNode m = merged;
    
    while(h1 != null && h2 != null) {
        if (h1.data < h2.data) {
            m.next = h1;
            h1 = h1.next;
        } else {
            m.next = h2;
            h2 = h2.next;
        }
        m = m.next;
    }
            
    if (h1 != null) m.next = h1;        
    if (h2 != null) m.next = h2;
    
    return merged.next;
}