Difference between revisions of "99 questions/Solutions/6"
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Dan.krejsa (talk | contribs) (Add a more efficient palindrome checker) |
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where |
where |
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input = zip xs (reverse xs) |
input = zip xs (reverse xs) |
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+ | </haskell> |
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+ | |||
+ | Here's one that does half as many compares: |
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+ | |||
+ | <haskell> |
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+ | palindrome :: (Eq a) => [a] -> Bool |
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+ | palindrome xs = p [] xs xs |
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+ | where p rev (x:xs) (_:_:ys) = p (x:rev) xs ys |
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+ | p rev (x:xs) [_] = rev == xs |
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+ | p rev xs [] = rev == xs |
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</haskell> |
</haskell> |
Revision as of 20:21, 20 May 2011
(*) Find out whether a list is a palindrome. A palindrome can be read forward or backward; e.g. (x a m a x).
isPalindrome :: (Eq a) => [a] -> Bool
isPalindrome xs = xs == (reverse xs)
isPalindrome' [] = True
isPalindrome' [_] = True
isPalindrome' xs = (head xs) == (last xs) && (isPalindrome' $ init $ tail xs)
Here's one to show it done in a fold just for the fun of it. Do note that it is less efficient then the previous 2 though.
isPalindrome'' :: (Eq a) => [a] -> Bool
isPalindrome'' xs = foldl (\acc (a,b) -> if a == b then acc else False) True input
where
input = zip xs (reverse xs)
Here's one that does half as many compares:
palindrome :: (Eq a) => [a] -> Bool
palindrome xs = p [] xs xs
where p rev (x:xs) (_:_:ys) = p (x:rev) xs ys
p rev (x:xs) [_] = rev == xs
p rev xs [] = rev == xs