生成随机数
**
// Swift 4.2
/ 1
let digit = Int.random(in: 0..<10)
// 2
if let anotherDigit = (0..<10).randomElement() {
print(anotherDigit)
} else {
print("Empty range.")
}
// 3
let double = Double.random(in: 0..<1)
let float = Float.random(in: 0..<1)
let cgFloat = CGFloat.random(in: 0..<1)
let bool = Bool.random()
注：randomElement() 如果 range 是空，返回 nil
数组随机
**
if let song = playlist.randomElement() {
print(song)
} else {
print("Empty playlist.")
}
洗牌算法
**
let shuffledPlaylist = playlist.shuffled()
names.shuffle()
