您可以使用
reduce(into:)方法（Swift4），如下所示：
let contacts = ["Aaron", "Adam", "Brian", "Brittany", ""]let dictionary = contacts.reduce(into: [String:[String]]()) { result, element in // make sure there is at least one letter in your string else return guard let first = element.first else { return } // create a string with that initial let initial = String(first) // initialize an array with one element or add another element to the existing value result[initial] = (result[initial] ?? []) + [element]}print(dictionary) // ["B": ["Brian", "Brittany"], "A": ["Aaron", "Adam"]]如果您使用的是Swift3或更早版本，则需要在闭包内部创建一个可变的结果字典：
let contacts = ["Aaron", "Adam", "Brian", "Brittany", ""]let dictionary = contacts.reduce([String:[String]]()) { result, element in var result = result guard let first = element.first else { return result } let initial = String(first) result[initial] = (result[initial] ?? []) + [element] return result }print(dictionary) // ["B": ["Brian", "Brittany"], "A": ["Aaron", "Adam"]]请注意，结果未排序。字典是无序集合。如果需要对字典进行排序并返回（键，值）元组的数组，则可以按以下方式使用按键排序：
let sorted = dictionary.sorted {$0.key < $1.key}print(sorted)“ [[（键：” A“，值：[” Aaron“，”亚当“]），（键：” B“，值：[” Brian“，” Brittany“]）] n”
