feat: add N-th Tribonacci number (#1202)

* add leetcode n-th Tribonacci number

* updating DIRECTORY.md

* updating DIRECTORY.md

---------

Co-authored-by: github-actions[bot] <github-actions@users.noreply.github.com>

leetcode/DIRECTORY.md

@@ -119,6 +119,7 @@
 | 1019 | [Next Greater Node In Linked List](https://leetcode.com/problems/next-greater-node-in-linked-list)                                                     | [C](./src/1019.c) | Medium     |
 | 1026 | [Maximum Difference Between Node and Ancestor](https://leetcode.com/problems/maximum-difference-between-node-and-ancestor)                             | [C](./src/1026.c) | Medium     |
 | 1089 | [Duplicate Zeros](https://leetcode.com/problems/duplicate-zeros)                                                                                       | [C](./src/1089.c) | Easy       |
+| 1137 | [N-th Tribonacci Number](https://leetcode.com/problems/n-th-tribonacci-number)                                                                         | [C](./src/1137.c) | Easy       |
 | 1147 | [Longest Chunked Palindrome Decomposition](https://leetcode.com/problems/longest-chunked-palindrome-decomposition)                                     | [C](./src/1147.c) | Hard       |
 | 1184 | [Distance Between Bus Stops](https://leetcode.com/problems/distance-between-bus-stops)                                                                 | [C](./src/1184.c) | Easy       |
 | 1189 | [Maximum Number of Balloons](https://leetcode.com/problems/maximum-number-of-balloons)                                                                 | [C](./src/1189.c) | Easy       |

leetcode/src/1137.c

@@ -0,0 +1,29 @@
+// Dynamic Programming
+// Runtime: O(n)
+// Space: O(1)
+int tribonacci(int n){
+    int t0 = 0;
+    int t1 = 1;
+    int t2 = 1;
+
+    if (n == 0) {
+        return t0;
+    }
+
+    if (n == 1){
+        return t1;
+    }
+
+    if (n == 2){
+        return t2;
+    }
+
+    for (int i = 0; i < n - 2; i++){
+        int nextT = t0 + t1 + t2;
+        t0 = t1;
+        t1 = t2;
+        t2 = nextT;
+    }
+
+    return t2;
+}