rs: add «1137. N-th Tribonacci Number»

Signed-off-by: Matej Focko <me@mfocko.xyz>

rs/n-th-tribonacci-number.rs

@@ -0,0 +1,17 @@
+impl Solution {
+    pub fn tribonacci(mut n: i32) -> i32 {
+        let mut seq = vec![0, 1, 1];
+
+        while n >= 3 {
+            let next = seq.iter().sum::<i32>();
+
+            seq[0] = seq[1];
+            seq[1] = seq[2];
+            seq[2] = next;
+
+            n -= 1;
+        }
+
+        seq[n as usize]
+    }
+}