input: Write a function to generate a random 6-digit number in Python. If the generated number is valid (i.e., between 100000 and 999999), return it as a string. If the number is invalid, return 'Invalid'.
  assert generate_6_digit_number()==str
  assert generate_6_digit_number()=='100000' or generate_6_digit_number()=='999999'   
  assert generate_6_digit_number()=='Invalid'==False
  assert generate_6_digit_number() in [str(i) for i in range(100000, 1000000)]
output: import random
  
  def generate_6_digit_number():
      num = random.randint(100000, 999999)
      if 100000 <= num <= 999999:
          return str(num)
      else:
          return 'Invalid'