A binary tree is univalued if every node in the tree has the same value.
Return true if and only if the given tree is univalued.
Example 1:
Input: [1,1,1,1,1,null,1] Output: true
Example 2:
Input: [2,2,2,5,2] Output: false
Note:
<li>The number of nodes in the given tree will be in the range <code>[1, 100]</code>.</li>
<li>Each node's value will be an integer in the range <code>[0, 99]</code>.</li>