C++

1class Solution {
2  public:
3    int rangeSumBST(TreeNode* root, int low, int high) {
4      if(!root) return 0;
5
6      int sum = 0;
7      if (root->val >= low && root->val <= high) sum = root->val;
8      if (root->val > low) sum += rangeSumBST(root->left, low, high);
9      if (root->val < high) sum += rangeSumBST(root->right, low, high);
10
11      return sum;
12    }
13};

JavaScript

1var rangeSumBST = function(root, low, high) {
2  if (!root) return 0;
3  let sum = 0;
4  if (root.val >= low && root.val <= high) sum = root.val;
5
6  return sum + rangeSumBST(root.left, low, high) + rangeSumBST(root.right, low, high);
7};

Ruby

1def range_sum_bst(root, low, high)
2  return 0 if root == nil
3  sum = 0
4
5  sum = root.val if (root.val >= low and root.val <= high)
6  sum += range_sum_bst(root.left, low, high) if root.val > low    
7  sum += range_sum_bst(root.right, low, high) if root.val < high
8
9  sum
10end