text: # Problem: Binary Tree Level Order Traversal
  #
  # Given a binary tree, return the level order traversal of its nodes' values.
  # (ie, from left to right, level by level).
  #
  # For example:
  #
  # Given binary tree [3,9,20,null,null,15,7],
  #    3
  #   / \
  #  9  20
  #    /  \
  #   15   7
  #
  # return its level order traversal as:
  #
  # [
  #  [3],
  #  [9,20],
  #  [15,7]
  # ]
  #
  ################################################################################
  