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class Solution {
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 public:
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  void sumGparent(TreeNode *child, int &sum, TreeNode *parent, TreeNode *Gparent) {
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    if (!child) return;
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    if (Gparent)
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      if (Gparent->val % 2 == 0) sum += child->val;
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    sumGparent(child->left, sum, child, parent);
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    sumGparent(child->right, sum, child, parent);
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  }
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  int sumEvenGrandparent(TreeNode *root) {
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    if (!root) return 0;
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    int sum = 0;
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    sumGparent(root, sum, NULL, NULL); // (child, sum, parent, grand-parent)
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    return sum;
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  }
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};