Hi,
Here is my solution to 4clojure problem #177:
Write a function that takes in a string and returns truthy if all square [
> ] round ( ) and curly { } brackets are properly paired and legally nested,
> or returns falsey otherwise.
>
(defn valid-parens?
[s]
(loop [[ch & chs] s stack []]
(if (nil? ch)
(empty? stack)
(case ch
(\( \[ \{) (recur chs (conj stack ch))
\) (if (= (peek stack) \()
(recur chs (pop stack))
false)
\] (if (= (peek stack) \[)
(recur chs (pop stack))
false)
\} (if (= (peek stack) \{)
(recur chs (pop stack))
false)
(recur chs stack)))))
This works but I don't like the repetition in the case branches. But the
repeated code contains a recur call so I can't simply refactor it into a
helper function. How can I achieves something like the following, but
without consuming the stack?
(defn valid-parens?
([s]
(valid-parens? s []))
([[ch & chs] stack]
(if (nil? ch)
(empty? stack)
(letfn [(match? [p]
(if (= (peek stack) p)
(valid-parens? chs (pop stack))
false))]
(case ch
(\( \[ \{) (valid-parens? chs (conj stack ch))
\) (match? \()
\] (match? \[)
\} (match? \{)
(valid-parens? chs stack))))))
Thanks!
Botond
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