创建单例的两种方法

class MyClass {
    static let singleton1 = MyClass()
    static let singleton2: MyClass = {
        let instance = MyClass()
        // setup code
        return instance
    }()
    private init() {}
}

[备注]：苹果官方推荐创建单例的方式

定义整型的平方

extension Int {
    var square: Int { self * self }
}
print(10.square)
print(10.square.square)

定义二维数组

struct D2Array {
    let rows: Int, colums: Int
    var grid: [Int]
    init(rows: Int, colums: Int) {
        self.rows = rows
        self.colums = colums
        grid = Array(repeating: 0, count: rows * colums)
    }
    subscript(row: Int, col: Int) -> Int{
        get {
            return grid[row * colums + col]
        }
        set {
            grid[row * colums + col] = newValue
        }
   }
}

跳出指定循环

label1: for x in 0..<5 {
    label2: for y in (0..<5).reversed() {
        if x == y {
            break label1 // break 后面指定这个标签，当条件成立就跳出该标签的外循环
        }
        print(x, y)
    }
}