難度：簡單。
標籤：樹，深度優先搜尋，遞迴。

正確解法：

/**
 * 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 {
    int ans = 0;
    void getResult(TreeNode* cur, int low, int high){
        if(cur == nullptr)return;
        if(cur->val >= low && cur->val <= high){
            ans += cur->val;
            getResult(cur->right, low, high);
            getResult(cur->
 
left, low, high);
        }
        else if(cur->val < low)getResult(cur->right, low, high);
        else if(cur->val > high)getResult(cur->left, low, high);
    }

public:
    int rangeSumBST(TreeNode* root, int low, int high) {
        getResult(root, low, high);
        return ans;
 

    }
};

結果：