input: Write a function to compute the nth Fibonacci number.
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(9) == 21
  assert fibonacci(10) == 34
  assert fibonacci(15) == 377
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      a, b = 0, 1
      for _ in range(2, n+1):
          a, b = b, a + b
      return b