根据Swift API Design Guidelines，使用Swift 3，NSJSONSerialization的名称及其方法已更改。

let dic = ["2": "B","1": "A","3": "C"]

do {
    let jsonData = try JSONSerialization.data(withJSONObject: dic,options: .prettyPrinted)
    // here "jsonData" is the dictionary encoded in JSON data

    let decoded = try JSONSerialization.jsonObject(with: jsonData,options: [])
    // here "decoded" is of type `Any`,decoded from JSON data

    // you can now cast it with the right type        
    if let dictFromJSON = decoded as? [String:String] {
        // use dictFromJSON
    }
} catch {
    print(error.localizedDescription)
}

Swift 2.x

do {
    let jsonData = try NSJSONSerialization.dataWithJSONObject(dic,options: NSJSONWritingOptions.PrettyPrinted)
    // here "jsonData" is the dictionary encoded in JSON data

    let decoded = try NSJSONSerialization.JSONObjectWithData(jsonData,options: [])
    // here "decoded" is of type `AnyObject`,decoded from JSON data

    // you can now cast it with the right type 
    if let dictFromJSON = decoded as? [String:String] {
        // use dictFromJSON
    }
} catch let error as NSError {
    print(error)
}

Swift 1

var error: NSError?
if let jsonData = NSJSONSerialization.dataWithJSONObject(dic,options: NSJSONWritingOptions.PrettyPrinted,error: &error) {
    if error != nil {
        println(error)
    } else {
        // here "jsonData" is the dictionary encoded in JSON data
    }
}

if let decoded = NSJSONSerialization.JSONObjectWithData(jsonData,options: nil,error: &error) as? [String:String] {
    if error != nil {
        println(error)
    } else {
        // here "decoded" is the dictionary decoded from JSON data
    }
}