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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def ToJsonString(self):
 """Converts Timestamp to RFC 3339 date string format. Returns: A string converted from timestamp. The string is always Z-normalized and uses 3, 6 or 9 fractional digits as required to represent the exact time. Example of the return format: '1972-01-01T10:00:20.021Z' """

    nanos = self.nanos % _NANOS_PER_SECOND
    total_sec = self.seconds + (self.nanos - nanos) // _NANOS_PER_SECOND
    seconds = total_sec % _SECONDS_PER_DAY
    days = (total_sec - seconds) // _SECONDS_PER_DAY
    dt = datetime(1970, 1, 1) + timedelta(days, seconds)

    result = dt.isoformat()
    if (nanos % 1e9) == 0:
      # If there are 0 fractional digits, the fractional
      # point '.' should be omitted when serializing.
      return result + 'Z'
    if (nanos % 1e6) == 0:
      # Serialize 3 fractional digits.
      return result + '.%03dZ' % (nanos / 1e6)
    if (nanos % 1e3) == 0:
      # Serialize 6 fractional digits.
      return result + '.%06dZ' % (nanos / 1e3)
    # Serialize 9 fractional digits.
    return result + '.%09dZ' % nanos