`toDictionary` by Extension of SequenceType ref: http://qiita.com/ryoma_kawajiri/items/83f13cc45396e221e24e

SequenceType+toDictionary.swift

extension SequenceType {
  func toDictionary<K, V>() -> [K: V] {
    var dictionary: [K: V] = [:]
    self.forEach { e in
      if let kv = e as? (K, V) {
        dictionary[kv.0] = kv.1
      }
    }
    return dictionary
  }

  func toDictionary<K, V>(transform: (element: Self.Generator.Element) -> (key: K, value: V)?) -> [K: V] {
    var dictionary: [K: V] = [:]
    self.forEach { e in
      guard let (key, value) = transform(element: e) else {
        return
      }
      dictionary[key] = value
    }
    return dictionary
  }

  func toDictionary<K, V>(combine: (value1: V, value2: V) -> V?) -> [K:  V] {
    var dictionary: [K: V] = [:]
    self.forEach { e in
      guard let kv = e as? (K, V) else {
        return
      }
      let (key, value2) = kv
      if let value1 = dictionary[key] {
        dictionary[key] = combine(value1: value1, value2: value2)
      } else {
        dictionary[key] = value2
      }
    }
    return dictionary
  }

  func toDictionary<K, V>(transform transform: (element: Self.Generator.Element) -> (key: K, value: V)?, combine: (value1: V, value2: V) -> V?) -> [K:  V] {
    var dictionary: [K: V] = [:]
    self.forEach { e in
      guard let (key, value2) = transform(element: e) else {
        return
      }
      if let value1 = dictionary[key] {
        dictionary[key] = combine(value1: value1, value2: value2)
      } else {
        dictionary[key] = value2
      }
    }
    return dictionary
  }
}