def mergeLists(head1, head2):

  new_list=SinglyLinkedList()

current1=head1
current2=head2

while current1 and current2:
    if current1.data==current2.data:
        new_list.insert_node(current1.data)
        new_list.insert_node(current2.data)
        current1=current1.next
        current2=current2.next
    elif current1.data < current2.data:
        new_list.insert_node(current1.data)
        current1=current1.next
    elif current2.data < current1.data:
        new_list.insert_node(current2.data)
        current2=current2.next

while current1:
    new_list.insert_node(current1.data)
    current1=current1.next

while current2:
    new_list.insert_node(current2.data)
    current2=current2.next

return new_list.head

result = SinglyLinkedList()
while head1 or head2:
    if not head1:
        result.insert_node(head2.data)
        head2 = head2.next
    elif not head2:
        result.insert_node(head1.data)
        head1 = head1.next
    elif head1 and head2:
        if head1.data <= head2.data:
            result.insert_node(head1.data)
            head1 = head1.next
        else:
            result.insert_node(head2.data)
            head2 = head2.next
return result.head

def mergeLists(head1, head2): dummy=SinglyLinkedListNode(0) cur=dummy while head1 and head2:

    if head1.data<=head2.data:
        cur.next=head1
        head1=head1.next
    else:
        cur.next=head2
        head2=head2.next
    cur=cur.next
if head1:
    cur.next=head1
if head2:
    cur.next=head2
return dummy.next

function mergeLists(head1, head2) { let newNode = new SinglyLinkedListNode(); let tail = newNode;

while(head1 !==null && head2 !== null){
    if(head1.data < head2.data){
        tail.next = head1;
        head1= head1.next;
    }else if(head2.data <= head1.data){
        tail.next = head2;
        head2 = head2.next;
    }
    tail = tail.next;


    if(head1 !==null){
        tail.next = head1;
    }else if(head2 !== null){
        tail.next = head2;
    }
}
    return newNode.next;

}

javascript:

function mergeLists(head1, head2) {
    let newnode = new SinglyLinkedListNode(0, null)
    let tempnode = newnode
    
    while(head1 !== null && head2 !== null ){
        if(head1.data < head2.data){
            tempnode.next = head1
            head1 = head1.next //inc of head1 linked list
        }else{
            tempnode.next = head2
            head2 = head2.next  //inc of head2 linked list
        }
        tempnode = tempnode.next //inc of newnode
    }
    
    tempnode.next = head1 || head2 // if either head1 is null then tempnode will update with head2 or vice-versa
    
    return newnode.next
}