var courses = [76:"慕课网2048私人订制",77:"CSS3 3D 特效",127:"玩儿转Swift"]
var websites = ["搜索引擎":"Google","学习网站":"immoc"]

var course2 = Dictionary<Int,String>()
var course3 = [:]
var course4 = Dictionary<String,String>()

courses.count//3
course2.isEmpty//true

courses[73]//nil

dictionary可以有isEmpty判断啊