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Hello everyone. Today is the Day 8 of the #100DaysOfCodeChallenge. Received a problem previously asked by Google with an easy tag to it.

## The Question On Day #8:

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
```

## Approach

- Before moving on to the main logic, let's discuss about the
**Brute Force**way to solve this:- Starting from the root, we check if the immediate children have the same value as the root node, and then the children of the immediate nodes and so on.
- We repeat this process for every subtree ending up in time taking process.

# Optimal Way

- The optimal way to solve this problem is by using the bottom-up approach. We first start from the leaf nodes, and move to the upper levels and use the unary value of the child subtree to decide if the main tree is a unary tree or not.

**Python Code**

**Output**

```
No. of unival subtrees= 5
```

# Visualiser

To understand the recursion behind this code, please click here

Feel free to reach out for any query clearance.

**Thanks and cheers:)**

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