斐波那契数列

• 递归生成斐波那契数列

  def fib(n:int):
      if n <2:
         return  n
      return fib(n-1) + fib(n-2)
  

• 给递归加个缓存

  cache = {}
  def fib(n:int):
    if n<2:
        return n
    result = cache.get(n,None)
    if result is None:
        cache[n] = fib(n-1) + fib(n-2)
        return cache[n]
    return result
  

• 创建斐波那契数列迭代器

  class Fib:
    def __init__(self,n:int):
        self.stop = n
        self.index = 1
        self.a,self.b = 0,1
  
    def __iter__(self):
        return self
  
    def __next__(self,):
        if self.index <= self.stop:
            self.a,self.b = self.b ,self.a+self.b
            self.index +=1
            return self.a
        raise StopIteration