Merge pull request neetcode-gh#3106 from nkawaller/n-th-tribonacci-number

Create 1137-n-th-tribonacci-number.cpp

Author: MHamiid

cpp/1137-n-th-tribonacci-number.cpp

@@ -0,0 +1,19 @@
+// Time: O(n)
+// Space: O(1)
+
+class Solution {
+public:
+    int tribonacci(int n) {
+        int t[] = {0, 1, 1};
+        if (n < 3) {
+            return t[n];
+        }
+        for (int i = 3; i <= n; i++) {
+            int sum_t = t[0] + t[1] + t[2];
+            t[0] = t[1];
+            t[1] = t[2];
+            t[2] = sum_t;
+        }
+        return t[2];
+    }
+};