function validateTopic (topic) {
  var parts = topic.split('/')

  for (var i = 0; i < parts.length; i++) {
    if (parts[i] === '+') {
      continue
    }

    if (parts[i] === '#') {
      // for Rule #2
      return i === parts.length - 1
    }

    if (parts[i].indexOf('+') !== -1 || parts[i].indexOf('#') !== -1) {
      return false
    }
  }

  return true
}
Validate a topic to see if it's valid or not.
A topic is valid if it follow below rules:
- Rule #1: If any part of the topic is not `+` or `#`, then it must not contain `+` and '#'
- Rule #2: Part `#` must be located at the end of the mailbox

@param {String} topic - A topic
@returns {Boolean} If the topic is valid, returns true. Otherwise, returns false.