N-th Tribonacci Number

export function tribonacci(n: number): number {
  if (n <= 2) {
    return Math.ceil(n / 2);
  }
 
  let [first, second, third] = [0, 1, 1];
  for (let i = 3; i <= n; i++) {
    const sum = first + second + third;
    [first, second, third] = [second, third, sum];
  }
 
  return third;
}