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

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

Example :

Input:  root = [5,1,5,5,5,null,5]

              5
             / \
            1   5
           / \   \
          5   5   5

Output: 4

[Tree] [Depth-First Search] [Binary Tree]

1. Subtree of Another Tree (Easy)
2. Longest Univalue Path (Medium)