added solution for 230. Kth Smallest Element in a BST

Author: n1i9c9k9

Diff for: C++/kth-smallest-element-in-a-bst.cpp

@@ -0,0 +1,35 @@
+/**
+ * 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 kthSmallest(TreeNode* root, int k) {
+        int count = 0;
+        return inorder(root, k, count);
+    }
+
+private:
+    int inorder(TreeNode* node, int k, int& count) {
+        //Base case: Zero node
+        if(!node) return -1;
+
+        //Traverse left branch for kth smallest value:
+        int leftVal = inorder(node->left, k, count);
+        if(count == k) return leftVal;
+
+        //Process root node:
+        ++count;
+        if(count == k) return node->val; 
+
+        //Traverse right branch for kth smallest value:
+        return inorder(node->right, k, count);
+    }
+};