我一直在尝试这个问题很长一段时间但是我没有做得很远。问题是要求生成一个字符串,其中来自输入字符串的所有重复字符都被该字符的单个实例替换。
例如,
(remove-repeats "aaaab") => "ab"
(remove-repeats "caaabb aa") => "cab a"
由于我试图使用累积递归来做到这一点,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。请帮我修改一下。
谢谢!!
答案 0 :(得分:4)
使用字符串有点烦人,所以我们将它包装在一个处理列表的工作函数中。通过这种方式,我们可以避免乱搞各地的转化。
(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))]))
答案 1 :(得分:1)
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)))