用 Playground 解一道三年级的奥数题

题目：由6，6，7，7，8，8六个数字组成的6位数中，有多少是能被168整除的？

//: Playground - noun: a place where people can play

import UIKit

func isValid(num : Int) -> Bool {
    var v = num
    var i6 = 0
    var i7 = 0
    var i8 = 0
    while(v > 0) {
        let tmp = v % 10
        if(tmp == 6) { i6+=1
        } else if(tmp == 7) { i7+=1
        } else if(tmp == 8) { i8+=1
        } else {
            return false
        }
        v = v/10
    }
    return (i6==2)&&(i7==2)&&(i8==2)
}

let start = Date(timeIntervalSinceNow: 0)

for i in 667788...887766  {
    if (i % 168 == 0) && isValid(num : i) {
        print(i)
    }
}

//把 ％ 替换为加法能够快一点点
//let startValue = 667788 + (168 - 667788 % 168)
//for i in stride(from: startValue, through: 887766, by: 168) {
//    if isValid(num: i) {
//        print(i)
//    }
//}

let end = Date(timeIntervalSinceNow: 0)

print(end.timeIntervalSince(start))

<blockquote>想想如果先执行isValid()， 运行时间会相差多少。再验证一下。</blockquote>