Count Same-Value Subtrees

Problem Statement

Given a binary tree, determine how many subtrees have all nodes containing the same value.

A subtree is considered uniform if every node in that subtree holds an identical value.

Examples

Example 1:
Input: root = [4,3,4,4,4,null,4]

          4
         / \
        3   4
       / \    \
      4   4    4

Output: 4

Here are the 4 uni-value subtrees from the input tree. 
Each of these subtrees has all nodes with the same value:

A single node with value 4 (rightmost leaf).
A single node with value 4 (left child of 3).
A single node with value 4 (right child of 3).
A subtree rooted at the right child of the root (4) with its own right child 4.

Example 2:
Input: root = [2,2,2,2,null,null,2]

          2
         / \
        2   2
       /     \
      2       2

Output: 5

Constraints

• The total number of nodes in the tree is between 1 and 5000.
• Node values are integers ranging from 0 to 100.