input: Write a python function to convert a decimal number to binary.
  assert dec_to_bin(10) == "1010"
  assert dec_to_bin(5) == "101"
  assert dec_to_bin(1) == "1"
  assert dec_to_bin(0) == ""
output: def dec_to_bin(n):
      if n == 0:
          return ""
      return dec_to_bin(n // 2) + str(n % 2)