const fib_it = n => {
  let [a, b] = [0, 1];
  while (n-- > 0) {
    [a, b] = [b, a + b];
  }
  return a;
}

const fib_rec = n => {
  if (n < 2) return n;
  return fib_rec(n-1) + fib_rec(n-2);
}

const fib_trec = n => {
  const imp = (acc, l, i) => {
    if (i < 1) return acc;
    return imp(acc + l, acc, i-1);
  }
  return imp(0, 1, n);
}