code: def fib_iter(n):
      
  
      # precondition
      assert n >= 0, 'n must be positive integer'
  
      fib_1 = 0
      fib_2 = 1
      sum = 0
      if n <= 1:
          return n
      for _ in range(n-1):
          sum = fib_1 + fib_2
          fib_1 = fib_2
          fib_2 = sum
      return sum
docs: [summary]
      Works iterative approximate O(n)
  
      Arguments:
          n {[int]} -- [description]
      
      Returns:
          [int] -- [description]