每日算法题: Print a tree level-by-level, with line-breaks

Hi, here's your problem today. This problem was recently asked by Apple:

You are given a tree, and your job is to print it level-by-level with linebreaks.

a
/ \
b c
/ \ / \
d e f g

The output should be
a
bc
defg
Here's a starting point:

from collections import deque



class Node(object):

  def __init__(self, val):

    self.val = val

    self.left = None

    self.right = None



  def __str__(self):

    # Fill this in.



tree = Node('a')

tree.left = Node('b')

tree.right = Node('c')

tree.left.left = Node('d')

tree.left.right = Node('e')

tree.right.left = Node('f')

tree.right.right = Node('g')



print tree

# a

# bc

# defg