从方案中的字符串中删除重复字符

Question

我已经尝试这个问题很长时间了，但并没有走得太远。问题是要求生成一个字符串，其中输入字符串中的所有重复字符都替换为该字符的单个实例。

例如，

(remove-repeats "aaaab") => "ab"
(remove-repeats "caaabb aa") => "cab a"

由于我正在尝试使用累积递归来做到这一点，所以到目前为止我有：

(define (remove-repeats s) 
  (local
    [(define (remove-repeats-acc s1 removed-so-far)
      (cond
        [(empty? (string->list s1))""]
        [else 
         (cond
           [(equal? (first (string->list s1)) (second (string->list s1))) 
         (list->string (remove-repeats-acc (remove (second (string->list s1)) (string->list s1)) (add1 removed-so-far)))]
           [else (list->string (remove-repeats-acc (rest (string->list s1)) removed-so-far))])]))]
    (remove-repeats-acc s 0)))

但这似乎不对。请帮我修改它以使其正常工作。

谢谢！！

Answer 1

使用字符串有点烦人，所以我们将它包裹在一个处理列表的工作函数中。这样我们就可以避免到处乱搞转换。

(define (remove-repeats str)
  (list->string (remove-repeats/list (string->list str))))

现在我们可以使用简单的递归来定义 remove-repeats/list 函数：

(define (remove-repeats/list xs)
  (cond
    [(empty? xs) xs]
    [(empty? (cdr xs)) xs]
    [(equal? (car xs) (cadr xs)) (remove-repeats/list (cdr xs))]
    [else (cons (car xs) (remove-repeats/list (cdr xs)))]))

这不是尾递归，但现在添加累加器应该更容易：

(define (remove-repeats str)
  (list->string (remove-repeats/list-acc (string->list str) '())))

(define (remove-repeats/list-acc xs acc)
  (cond
    [(empty? xs) (reverse acc)]
    [(empty? (cdr xs)) (reverse (cons (car xs) acc))]
    [(equal? (car xs) (cadr xs)) (remove-repeats/list-acc (cdr xs) acc)]
    [else (remove-repeats/list-acc (cdr xs) (cons (car xs) acc))]))

Answer 2

这是我喜欢的版本，Typed Racket：

#lang typed/racket
(: remove-repeats : String -> String)
(define (remove-repeats s)
  (define-values (chars last)
    (for/fold: ([chars : (Listof Char) null] [last : (Option Char) #f])
      ([c (in-string s)] #:when (not (eqv? last c)))
      (values (cons c chars) c)))
  (list->string (reverse chars)))