直接上程式碼，都是很簡單的進位制轉換演算法

final class Conversion {
    
    // MARK: - 十進位制轉二進位制
    class func decTobin(number:Int) -> String {
        var num = number
        var str = ""
        while num > 0 {
            str = "\(num % 2)" + str
            num /= 2
        }
        return str
    }
    
    // MARK: - 二進位制轉十進位制
    class func binTodec(number num: String) -> Int {
        var sum: Int = 0
        for c in num.characters {
            let str = String(c)
            sum = sum * 2 + Int(str)!
        }
        return sum
    }
    
    // MARK: - 十進位制轉十六進位制
    class func decTohex(number:Int) -> String {
        return String(format: "%0X", number)
    }
    
    // MARK: - 十六進位制轉十進位制
    class func hexTodec(number num:String) -> Int {
        let str = num.uppercased()
        var sum = 0
        for i in str.utf8 {
            sum = sum * 16 + Int(i) - 48 // 0-9 從48開始
            if i >= 65 {                 // A-Z 從65開始，但有初始值10，所以應該是減去55
                sum -= 7
            }
        }
        return sum
    }
}
 

print(Conversion.decTobin(number: 4))
print(Conversion.binTodec(number: "100"))
print(Conversion.decTohex(number: 16))
print(Conversion.hexTodec(number: "10"))

//系統自帶
print(String(16, radix: 16))
print(String(16, radix: 8))
print(String(16, radix: 2))
print(String(16, radix: 10))