祖父节点值为偶数的节点和

// Java solution using Java Streams for finding the sum of nodes with even-valued grandparent

/*
 * Approach:
 * 1. Flatten the tree nodes into a list using DFS.
 * 2. For each node, check if it has a grandparent and if the grandparent's value is even.
 * 3. Use Java Streams to filter nodes with even-valued grandparents and sum their values.
 */

import java.util.*;
import java.util.stream.*;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

public class Solution {
    public int sumEvenGrandparent(TreeNode root) {
        List<TreeNode> nodeList = new ArrayList<>();
        dfs(root, null, null, nodeList);
        return nodeList.stream()
                .filter(node -> node[2] != null && node[2].val % 2 == 0)
                .mapToInt(node -> node[0].val)
                .sum();
    }

    private void dfs(TreeNode node, TreeNode parent, TreeNode grandparent, List<TreeNode[]> nodeList) {
        if (node == null) return;
        nodeList.add(new TreeNode[]{node, parent, grandparent});
        dfs(node.left, node, parent, nodeList);
        dfs(node.right, node, parent, nodeList);
    }
}