feat: add Kth Smallest Element in a BST (#1157)

Co-authored-by: David Leal <halfpacho@gmail.com>

leetcode/DIRECTORY.md

@@ -61,6 +61,7 @@
 | 217  | [Contains Duplicate](https://leetcode.com/problems/contains-duplicate/)                                                         | [C](./src/217.c)  | Easy       |
 | 223  | [Rectangle Area](https://leetcode.com/problems/rectangle-area/)                                                                 | [C](./src/223.c)  | Medium     |
 | 226  | [Invert Binary Tree](https://leetcode.com/problems/invert-binary-tree/)                                                         | [C](./src/226.c)  | Easy       |
+| 230  | [Kth Smallest Element in a BST](https://leetcode.com/problems/kth-smallest-element-in-a-bst/)                                   | [C](./src/230.c)  | Medium     |
 | 231  | [Power of Two](https://leetcode.com/problems/power-of-two/)                                                                     | [C](./src/231.c)  | Easy       |
 | 234  | [Palindrome Linked List](https://leetcode.com/problems/palindrome-linked-list/)                                                 | [C](./src/234.c)  | Easy       |
 | 242  | [Valid Anagram](https://leetcode.com/problems/valid-anagram/)                                                                   | [C](./src/242.c)  | Easy       |

leetcode/src/230.c

@@ -0,0 +1,35 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     struct TreeNode *left;
+ *     struct TreeNode *right;
+ * };
+ */
+
+struct TreeNode* findKthSmallest(struct TreeNode* node, int* k){
+    if (node == NULL){
+        return NULL;
+    }
+    
+    struct TreeNode* resultNode = findKthSmallest(node->left, k);
+    
+    if (resultNode != NULL){
+        return resultNode;
+    }
+    
+    *k -= 1;
+
+    if (*k == 0){
+        return node;
+    }
+    
+    return findKthSmallest(node->right, k);
+}
+
+// Depth-First Search
+// Runtime: O(n)
+// Space: O(1)
+int kthSmallest(struct TreeNode* root, int k){
+    return findKthSmallest(root, &k)->val;
+}