cpp: add “938. Range Sum of BST”

Signed-off-by: Matej Focko <mfocko@redhat.com>

cpp/range-sum-of-bst.cpp

@@ -0,0 +1,28 @@
+/**
+ * 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));
+    }
+};