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:
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 number of the node in the tree will be in the range
[0, 1000]. -1000 <= Node.val <= 1000