Daily Interview Problem: Count Number of Unival Subtrees

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

A unival tree is a tree where all the nodes have the same value. Given a binary tree, return the number of unival subtrees in the tree.

For example, the following tree should return 5:

   0

  / \

 1   0

    / \

   1   0

  / \

 1   1

The 5 trees are:
- The three single '1' leaf nodes. (+3)
- The single '0' leaf node. (+1)
- The [1, 1, 1] tree at the bottom. (+1)

Here's a starting point:

class Node(object):

  def __init__(self, val):

    self.val = val

    self.left = None

    self.right = None



def count_unival_subtrees(root):

  # Fill this in.



a = Node(0)

a.left = Node(1)

a.right = Node(0)

a.right.left = Node(1)

a.right.right = Node(0)

a.right.left.left = Node(1)

a.right.left.right = Node(1)



print count_unival_subtrees(a)

# 5