added Kth-smallest-element-in-a-bst problem

Binary Search Tree/Kth-smallest-element-in-a-bst.cpp

@@ -0,0 +1,29 @@
+/**
+ * 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:
+    void inorder(TreeNode * root, int &currentIndex,int k, int &value){
+        if(root){  
+            inorder(root->left, currentIndex, k, value);
+            currentIndex = currentIndex+1;
+            if(currentIndex==k){
+                value = root->val;
+            }
+            inorder(root->right, currentIndex, k, value);
+        }
+    }
+    int kthSmallest(TreeNode* root, int k) {
+        int currentIndex = 0, value = -1;
+        inorder(root, currentIndex, k, value);
+        return value;
+    }
+};