types - 在 F# 中实现队列类型

Question

我正在尝试在 F# 中实现一个队列，到目前为止，这就是我所拥有的，但我认为它的行为更像是一个堆栈:

type 'a queue = NL| Que of 'a * 'a queue;;

let enque m = function
    |NL -> Que(m, NL)
    |Que(x, xs) -> Que(m, Que(x, xs));;

let rec peek  = function
    |NL -> failwith "queue is empty"
    |Que(x, xs) -> x;;

let rec deque  = function
    |NL -> failwith "queue is empty"
    |Que(x, xs) -> xs;;

let rec build = function
    | [] -> NL
    | x::xs -> enque x (build xs);;

除了 enque 之外，其他操作都工作正常，我想让它能够将新元素添加到队列的后面而不是前面。

Answer 1

功能队列的规范方法是拥有两个列表，这会导致摊销 O(1) 访问:

type queue<'a> =
    | Queue of 'a list * 'a list

let empty = Queue([], [])

let enqueue q e = 
    match q with
    | Queue(fs, bs) -> Queue(e :: fs, bs)

let dequeue q = 
    match q with
    | Queue([], []) -> failwith "Empty queue!"
    | Queue(fs, b :: bs) -> b, Queue(fs, bs)
    | Queue(fs, []) -> 
        let bs = List.rev fs
        bs.Head, Queue([], bs.Tail)