直接暴力递归

/**
 * 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 rangeSumBST(TreeNode* root, int low, int high) {
        if(!root) return 0;
        int t = root->val;
        return t * (t >= low && t <= high) + rangeSumBST(root->left , low , high) + rangeSumBST(root->right , low , high);
    }
};

加一点小优化

/**
 * 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 rangeSumBST(TreeNode* root, int low, int high) {
        if(!root) return 0;
        int t = root->val;
        if(t < low) return rangeSumBST(root->right , low , high);//若当前值小于规定的最小值，只能遍历右子树
        else if(t > high) return rangeSumBST(root->left , low , high);//若当前值大于规定最大值，只能遍历左子树
        else return t + rangeSumBST(root->left , low , high) + rangeSumBST(root->right , low , high);//否则两边都得遍历
    }
};