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.

Solution: Recursion

Time complexity: O(n)
Space complexity: O(n)

// Author: Huahua
class Solution {
public:
  int sumEvenGrandparent(TreeNode* root, int p = 1, int gp = 1) {    
    if (!root) return 0;
    return sumEvenGrandparent(root->left, root->val, p)
           + sumEvenGrandparent(root->right, root->val, p) 
           + (gp & 1 ? 0 : root->val);
  }
};

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