最开始看到这题想到用BFS实现，写完后才发现不行，如果当前行存在偶数，我下面的代码，会把下下行的结点加进去。

/**
 * 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 {
public:
    int sumEvenGrandparent(TreeNode* root) {
        //BFS
        if(root == NULL) return 0;
        queue<TreeNode *> queue;
        int sum = 0,depth = 0;
        queue.push(root);
        unordered_set<int> hash;
        
        while(!queue.empty())
        {
            int size = queue.size();
            for(int i = 0;i < size;i++)
            {
                TreeNode* grand = queue.front();
                queue.pop();
                if(hash.count(depth))  
                {
                    sum += grand->val;
                }
                
                if(grand->val %2 == 0) hash.insert(depth + 2);
                
                if(grand->left)  queue.push(grand->left);
                if(grand->right)  queue.push(grand->right);
            }
            depth += 1;
        }
        
        return sum;
    }
};

修改后可AC的代码如下:

/**
 * 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 {
public:
    int sumEvenGrandparent(TreeNode* root) {
        //BFS
        if(root == NULL) return 0;
        queue<TreeNode *> queue;
        int sum = 0;
        queue.push(root);
        
        while(!queue.empty())
        {
            int size = queue.size();
            for(int i = 0;i < size;i++)
            {
                TreeNode* grand = queue.front();
                queue.pop();
                
                if(grand->val %2 == 0)
                {
                    if(grand->left)
                    {
                        sum += get_node_value(grand->left->left);
                        sum += get_node_value(grand->left->right);
                    }
                    
                    if(grand->right)
                    {
                        sum += get_node_value(grand->right->left);
                        sum += get_node_value(grand->right->right);
                    }
                }
                
                if(grand->left)  queue.push(grand->left);
                if(grand->right)  queue.push(grand->right);
            }
        }
        
        return sum;
    }
private:
    int get_node_value(TreeNode *root)
    {
        if(root == NULL) return 0;
        return root->val;
    }
};