let numbers = [1, 2, 3, 4, 5, 6, 7, 8]
        let multiply: (Int, Int) -> Int = {x, y in x * y }
        let total1 = numbers.reduce(3, multiply)
        print(total1) // 120960
        
        let total2 = numbers.reduce(3, {$0 * $1} )
        print(total2) // 120960
        
        let total3 = numbers.reduce(3, {(x: Int, y: Int) in
            x * y
        })
        print(total3) // 120960
        let total4 = numbers.reduce(3, {(x, y) in
            x * y
        })
        print(total4) // 120960

reduce 取出數(shù)組中每一個元素，進(jìn)行閉包中的操作，并返回與下一個元素進(jìn)行閉包中的操作，直至結(jié)束，返回最終值，以上三種閉包用法都可以。

        let numbers = [1, 2, 3, 4]
        let mapped = numbers.map { Array(repeating: $0, count: $0) }
        print(mapped) // [[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]]
        let flatMapped = numbers.flatMap { Array(repeating: $0, count: $0) }
        print(flatMapped) // [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]

map 取出數(shù)組中的每一個元素，進(jìn)行閉包中的操作，然后返回一個元素，組成新的數(shù)組
Array(repeating: 0) 返回一個數(shù)組，重復(fù)repeating count次
例如Array(repeating: 9, count: 3) 返回[9，9，9]
flatMap 取出數(shù)組中的元素，進(jìn)行閉包中的變換，返回為一個single-level數(shù)組，并且會將nil值去掉

        let cast = ["Vivien", "Marlon", "Kim", "Karl"]
        let shortNames = cast.filter { $0.characters.count < 5 }
        print(shortNames) // ["Kim", "Karl"]

filter 過濾集合中的元素。返回符合條件的集合