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# 0250. Count Univalue Subtrees

<https://leetcode.com/problems/count-univalue-subtrees>

## Description

Given the `root` of a binary tree, return the number of **uni-value** subtrees.

A **uni-value subtree** means all nodes of the subtree have the same value.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/08/21/unival_e1.jpg)

```
**Input:** root = [5,1,5,5,5,null,5]
**Output:** 4
```

**Example 2:**

```
**Input:** root = []
**Output:** 0
```

**Example 3:**

```
**Input:** root = [5,5,5,5,5,null,5]
**Output:** 6
```

**Constraints:**

* The numbrt of the node in the tree will be in the range `[0, 1000]`.
* `-1000 <= Node.val <= 1000`

## ac

```java
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int cnt;

    public int countUnivalSubtrees(TreeNode root) {
        cnt = 0;
        helper(root);
        return cnt;
    }

    private boolean helper(TreeNode root) {
        // exit
        if (root == null) return true;
        if (root.left == null && root.right == null) {
            cnt++;
            return true;
        }

        // handle
        // subtree is uni-value?
        boolean left = helper(root.left);
        boolean right = helper(root.right);
        if (!left || !right) {
            return false;
        }

        // root value equals children value?
        int lval = root.left != null ? root.left.val : root.val;
        int rval = root.right != null ? root.right.val : root.val;
        if (root.val == lval && root.val == rval) {
            cnt++;
            return true;
        }

        // unequal, return false
        return false;
    }
}
```