input: Write a python function to convert a decimal number to binary and vice versa.
  assert decimal_to_binary(15) == "1111"
  assert binary_to_decimal("1111") == 15
  assert decimal_to_binary(8) == "1000"
  assert binary_to_decimal("1000") == 8
output: def decimal_to_binary(n):
      return bin(n)[2:]
  
  def binary_to_decimal(b):
      return int(b, 2)