function mergeLists(head1, head2) {    
    var arr = []
    var cur = head1
    while(cur != null){
        arr.push(cur.data)
        cur = cur.next
    }
    
    cur = head2
    while(cur != null){
        arr.push(cur.data)
        cur = cur.next
    }
            
    arr.sort((a, b) => a - b) 
    
    var ll = new SinglyLinkedList()
    for(var i = 0; i < arr.length; i++){
        ll.insertNode(arr[i]) 
    }
    
    console.log(ll)
    
    return ll.head
}

    static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
        
        
        SinglyLinkedListNode head = new SinglyLinkedListNode(0); 
        SinglyLinkedListNode temp = head; //temp head, use for traversal
        SinglyLinkedListNode node1 = head1;
        SinglyLinkedListNode node2 = head2;
        while(node1 != null && node2 != null){
            if(node1.data < node2.data){
                temp.next = node1;
                node1 = node1.next;
            }else{
                temp.next = node2;
                node2 = node2.next;
            }
            temp = temp.next;
        }
        
        if(node1 != null){
            temp.next = node1;
        }
        
        if(node2 != null){
            temp.next = node2;
        }
        
        return head.next;
    }

sys.setrecursionlimit(2000)

def mergeLists(head1, head2):
    if not (head1 and head2):
        return head1 if head1 else head2

    if head1.data < head2.data:
        head1.next = mergeLists(head1.next, head2)
        return head1
    else:
        head2.next = mergeLists(head1, head2.next)
        return head2

static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
    SinglyLinkedListNode head = null;
    SinglyLinkedListNode tail = null;
        
    while( head1!=null || head2!=null) {
        SinglyLinkedListNode t = null ;
        if ( head2 == null || (head1!=null && head1.data <= head2.data) ) {
            t = head1;
            head1 = head1.next;
        } else {
            t = head2 ;
            head2 = head2.next;                
        }

        if (tail != null )
            tail.next = t;
        tail = t;
        if (head == null)
            head = tail;
    }
        
    tail.next = null;
    return head;
}