Hi, here’s your problem today. This problem was recently asked by Twitter:

Given a binary tree and an integer k, filter the binary tree such that its leaves don’t contain the value k. Here are the rules:

• If a leaf node has a value of k, remove it.
• If a parent node has a value of k, and all of its children are removed, remove it.

Here’s an example and some starter code:

class Node:
  def __init__(self, value, left=None, right=None):
    self.value = value
    self.left = left
    self.right = right

  def __repr__(self):
    return f"value: {self.value}, left: ({self.left.__repr__()}), right: ({self.right.__repr__()})"

def filter(tree, k):
  # Fill this in.

#     1
#    / \
#   1   1
#  /   /
# 2   1
n5 = Node(2)
n4 = Node(1)
n3 = Node(1, n4)
n2 = Node(1, n5)
n1 = Node(1, n2, n3)

print(filter(n1, 1))
#     1
#    /
#   1
#  /
# 2
# value: 1, left: (value: 1, left: (value: 2, left: (None), right: (None)), right: (None)), right: (None)