A binary tree is uni-valued if every node in the tree has the same value.

Given the `root` of a binary tree, return `true` if the given tree is uni-valued, or `false` otherwise.

Example 1:

    Input: root = [1,1,1,1,1,null,1]
    Output: true

Example 2:

    Input: root = [2,2,2,5,2]
    Output: false

Constraints:

- The number of nodes in the tree is in the range `[1, 100]`.
- `0 <= Node.val < 100`