AttributeError: 'NoneType' object has no attribute 'append'
    queue[depth+1] = queue.get(depth+1,[]).append(node.right)
Line 23 in levelOrder (Solution.py)
    ret = Solution().levelOrder(param_1)
Line 43 in _driver (Solution.py)
    _driver()
Line 54 in <module> (Solution.py)

if node.left:
    queue[depth+1] = queue.get(depth+1,[]).append(node.left)

if node.left:
    queue[depth+1] = queue.get(depth+1,[])
    queue[depth+1].append(node.left)