cs: add “1137. N-th Tribonacci Number”

Signed-off-by: Matej Focko <mfocko@redhat.com>

cs/n-th-tribonacci-number.cs

@@ -0,0 +1,18 @@
+public class Solution {
+    public int Tribonacci(int n) {
+        var sequence = new int[]{0, 1, 1};
+
+        if (n < 3) {
+            return sequence[n];
+        }
+
+        for (var i = 3; i <= n; ++i) {
+            var next = sequence.Sum();
+            sequence[0] = sequence[1];
+            sequence[1] = sequence[2];
+            sequence[2] = next;
+        }
+
+        return sequence[2];
+    }
+}