input: Write a function to print the Fibonacci series up to n numbers, where n is provided as input. If the input is less than or equal to 0, print "Invalid input".
  assert fibonacci_series(10) == "0 1 1 2 3 5 8 13 21 34"
  assert fibonacci_series(1) == "0"
  assert fibonacci_series(0) == "Invalid input"
  assert fibonacci_series(-5) == "Invalid input"
output: def fibonacci_series(n):
      if n <= 0:
          return "Invalid input"
      if n == 1:
          return "0"
      result = ["0", "1"]
      a, b = 0, 1
      for _ in range(2, n):
          a, b = b, a + b
          result.append(str(b))
      return " ".join(result)