input: Write a function to create and traverse a multi-level linked list.
  assert [node.data for node in multiLevelLL(root)] == [10, 2, 3, 6, 5, 8, 19, 4, 11, 7]
  assert [node.data for node in multiLevelLL(Node(1))] == [1]
  assert [node.data for node in multiLevelLL(None)] == []
output: class Node:
      def __init__(self, data, next=None, child=None):
          self.data = data
          self.next = next
          self.child = child
  
  def multiLevelLL(root):
      if not root:
          return []
      
      result = []
      stack = [root]
      
      while stack:
          current = stack.pop()
          while current:
              result.append(current)
              if current.child:
                  stack.append(current.child)
              current = current.next
              
      return result