def traverse(self, root):
    result = []
    # TODO: Write your code here
    level = []
    q = [root]

    levelSize = len(q)
    while len(q) > 0:
      node = q.pop(0)
      level.append(node.val)

      if node.left:
        q.append(node.left)

      if node.right:
        q.append(node.right)
      
      if len(level) == levelSize:
        result.append(level)
        level = []
        levelSize = len(q)

    return result