Count Univalue Subtrees

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

A subtree of treeName is a tree consisting of a node in treeName and all of its descendants.

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