題目：

思路：

呃，我知道要用DFS，但乍看題目看不懂什麼是even-value的意思，所以只好看Hint跟Discussion
　
　
　

重新翻譯題目

二元樹中，如果現在節點的上上層節點的值是偶數，就把現在節點的值sum起來
　
　

程式碼

class Solution {
public:
    int sumEvenGrandparent(TreeNode* root) {
        return sum(root, NULL, NULL); // root沒父沒祖母
    }
private:
    int sum(TreeNode* curr_node, TreeNode* parent, TreeNode* grandParent) {
        if (curr_node == NULL) {
            return 0;
        }
        int addition = 0;
        if (grandParent != NULL) 
            addition = curr_node->val;
        
        TreeNode* Im_even_node= NULL;
        if (curr_node->val % 2 == 0)
            Im_even_node = curr_node;

        return addition + sum(curr_node->left, Im_even_node, parent) + sum(curr_node->right, Im_even_node, parent);
    }

};

參考：
https://leetcode.com/problems/sum-of-nodes-with-even-valued-grandparent/discuss/477095/Easy-DFS-solution