Sum of Nodes with Even-Valued Grandparent

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:

• The number of nodes in the tree is between 1 and 10^4.

• The value of nodes is between 1 and 100.

class Solution {
    public static int sum = 0;

    public int sumEvenGrandparent(TreeNode root) {
        sum=0;
        dfs(root, null, null);
        return sum;
    }

    void dfs(TreeNode current, TreeNode parent, TreeNode grandParent) {
        if (current == null)
            return; // base case

        if (grandParent != null && grandParent.val % 2 == 0)
            sum += current.val;
        
        dfs(current.left, current, parent);// ( newChild, parent, GrandParent)
        dfs(current.right, current, parent);
    }
}