Given a binary tree, return the sum of values of nodes with even-valued grandparent. (A grandparent of a node is the parent of its parent, if it exists.)
If there are no nodes with an even-valued grandparent, return 0.
Example 1:
Input: root = [6,7,8,2,7,1,3,9,null,1,4,null,null,null,5] Output: 18 Explanation: The red nodes are the nodes with even-value grandparent while the blue nodes are the even-value grandparents.
Constraints:
<li>The number of nodes in the tree is between <code>1</code> and <code>10^4</code>.</li>
<li>The value of nodes is between <code>1</code> and <code>100</code>.</li>