input: Write a python function to convert a decimal number to binary.
  assert decimal_to_binary(5) == '101'
  assert decimal_to_binary(10) == '1010'
  assert decimal_to_binary(0) == '0'
  assert decimal_to_binary(15) == '1111'
output: def decimal_to_binary(n):
      if n == 0:
          return '0'
      binary = ''
      while n > 0:
          binary = str(n % 2) + binary
          n = n // 2
      return binary