message: Update print-binary-tree.py
diff: # Time: O(n)
  # Space: O(h)
  +# Print a binary tree in an m*n 2D string array following these rules:
  +#
  +# The row number m should be equal to the height of the given binary tree.
  +# The column number n should always be an odd number.
  +# The root node's value (in string format) should be put in the exactly middle of the first row it can be put.
  +# The column and the row where the root node belongs will separate the rest space into two parts
  +# (left-bottom part and right-bottom part). You should print the left subtree in the left-bottom part and
  +# print the right subtree in the right-bottom part. The left-bottom part and the right-bottom part should have the same size.
  +# Even if one subtree is none while the other is not, you don't need to print anything for the none subtree
  +# but still need to leave the space as large as that for the other subtree. However, if two subtrees are none,
  +# then you don't need to leave space for both of them.
  +# Each unused space should contain an empty string "".
  +# Print the subtrees following the same rules.
  +# Example 1:
  +# Input:
  +# 1
  +# /
  +# 2
  +# Output:
  +# [["", "1", ""],
  +# ["2", "", ""]]
  +# Example 2:
  +# Input:
  +# 1
  +# / \
  +# 2 3
  +# \
  +# 4
  +# Output:
  +# [["", "", "", "1", "", "", ""],
  +# ["", "2", "", "", "", "3", ""],
  +# ["", "", "4", "", "", "", ""]]
  +# Example 3:
  +# Input:
  +# 1
  +# / \
  +# 2 5
  +# /
  +# 3
  +# /
  +# 4
  +# Output:
  +#
  +# [["", "", "", "", "", "", "", "1", "", "", "", "", "", "", ""]
  +# ["", "", "", "2", "", "", "", "", "", "", "", "5", "", "", ""]
  +# ["", "3", "", "", "", "", "", "", "", "", "", "", "", "", ""]
  +# ["4", "", "", "", "", "", "", "", "", "", "", "", "", "", ""]]
  +# Note: The height of binary tree is in the range of [1, 10].
  +#
  # Definition for a binary tree node.
  # class TreeNode(object):
  # def __init__(self, x):
  