leetcode 1315. 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.

题目大意：计算树中所有祖父结点值为偶数的结点值的和。

方法一：直接用深搜的递归做法：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
private:
    int dfs(TreeNode *root, TreeNode *parent, TreeNode *grandParent) {
        if (root == nullptr) return 0;
        int sum = 0;
        if ((grandParent != nullptr) && ((grandParent->val & 1) == 0))
            sum = root->val;
        return sum + dfs(root->left, root, parent) + dfs(root->right, root, parent);
    }
public:
    int sumEvenGrandparent(TreeNode* root) {
        if (root == nullptr)
            return 0;
        return dfs(root, nullptr, nullptr);
    }
};