/**
 * 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 == nullptr) {
            return 0;
        }

        int currentValue = 0;
        if (low <= root->val && root->val <= high) {
            currentValue = root->val;
        }

        return (currentValue + rangeSumBST(root->left, low, high) +
                rangeSumBST(root->right, low, high));
    }
};