input: Write a python function to return the nth Fibonacci number.
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(10) == 55
  assert fibonacci(20) == 6765
  assert fibonacci(50) == 12586269025
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