# Problem 8

## Description

This problem was asked by Google.

A unival tree (which stands for "universal value") is a tree where all nodes under it have the same value.

Given the root to a binary tree, count the number of unival subtrees.

For example, the following tree has 5 unival subtrees:

```
0
/ \
1 0
/ \
1 0
/ \
1 1
```