input: 
  def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)
  
  n = 10
  print(fibonacci(n))
  
  # Test case:
  # >>> fibonacci(10)
  # 55
  
output:     a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b