input: Write a python function to check if a number is an Armstrong number.
  assert is_armstrong_number(153) == True
  assert is_armstrong_number(370) == True
  assert is_armstrong_number(9474) == False
output: def is_armstrong_number(n):
      num_str = str(n)
      num_len = len(num_str)
      total = sum(int(digit) ** num_len for digit in num_str)
      return total == n