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
}