feat: add Trim a Binary Search Tree LeetCode problem (#1156)

Author: alexpantyukhin

leetcode/DIRECTORY.md

@@ -80,6 +80,7 @@
 | 561  | [Array Partition I](https://leetcode.com/problems/array-partition-i/)                                                           | [C](./src/561.c)  | Easy       |
 | 617  | [Merge Two Binary Trees](https://leetcode.com/problems/merge-two-binary-trees/)                                                 | [C](./src/617.c)  | Easy       |
 | 647  | [Palindromic Substring](https://leetcode.com/problems/palindromic-substrings/)                                                  | [C](./src/647.c)  | Medium     |
+| 669  | [Trim a Binary Search Tree](https://leetcode.com/problems/trim-a-binary-search-tree/)                                           | [C](./src/669.c)  | Medium     |
 | 674  | [Longest Continuous Increasing Subsequence](https://leetcode.com/problems/longest-continuous-increasing-subsequence/)           | [C](./src/674.c)  | Easy       |
 | 700  | [Search in a Binary Search Tree](https://leetcode.com/problems/search-in-a-binary-search-tree/)                                 | [C](./src/700.c)  | Easy       |
 | 701  | [Insert into a Binary Search Tree](https://leetcode.com/problems/insert-into-a-binary-search-tree/)                             | [C](./src/701.c)  | Medium     |

leetcode/src/669.c

@@ -0,0 +1,30 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     struct TreeNode *left;
+ *     struct TreeNode *right;
+ * };
+ */
+
+
+// Depth-First Search
+// Runtime: O(n)
+// Space: O(1)
+struct TreeNode* trimBST(struct TreeNode* root, int low, int high){
+    if (root == NULL){
+        return NULL;
+    }
+    
+    if (root->val > high){
+        return trimBST(root->left, low, high);
+    }
+
+    if (root->val < low){
+        return trimBST(root->right, low, high);
+    }
+    
+    root->left = trimBST(root->left, low, high);
+    root->right = trimBST(root->right, low, high);
+    return root;
+}