利⽤ Swift 语⾔，实现⼀个⽇期 Date 类型的 API
各位程序猿能不能帮忙看看，怎么做？求帮助了！！
问题相关代码
struct Date: Comparable, CustomStringConvertible {
private static let DAYS = [0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
let year: Int
let month: Int
let day: Int
var description: String {
}
init?(date: String){
}
init?(month: Int, day: Int, year: Int){
}
func next() -> Date {
}
private static func isLeapYear(year y: Int) -> Bool {
}
private static func isValid(_ m: Int, _ d: Int, _ y: Int) -> Bool {
}
func isAfter(date: Date) -> Bool {
}
