input: Write a function to add nodes to a tree and traverse it using a Python dictionary.
  assert add_node_and_traverse('root', 'child1', 'child2') == ['root-->child1', 'root-->child2']
  assert add_node_and_traverse('A', 'B', 'C') == ['A-->B', 'A-->C']
  assert add_node_and_traverse('X', 'Y', 'Z') == ['X-->Y', 'X-->Z']
output: def add_node_and_traverse(root, child1, child2):
      tree = {root: [child1, child2]}
      return [f"{root}-->{child}" for child in tree[root]]