input: Write a python function to generate the nth Fibonacci number.
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(2) == 1
  assert fibonacci(3) == 2
  assert fibonacci(4) == 3
  assert fibonacci(5) == 5
  assert fibonacci(10) == 34
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          a, b = 0, 1
          for _ in range(2, n+1):
              a, b = b, a + b
          return b