Swift基础---常量、变量

let constants = 10
constants = 20 ----> error

var x = 0
var a = 1, b = 2, c = 3
x = 10

var welcomeMessage: String

let π = 3.14159
let 你好 = "你好世界"
let ?? = "dogcow”

println(x)
println(constants)