Difference between revisions of "99 questions/Solutions/82"

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(categorize)
(list as monad)
 
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Brute-force search from the source, using list comprehension:
 
Brute-force search from the source, using list comprehension:
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<haskell>
 
import Data.List (partition)
 
import Data.List (partition)
 
cycle' :: Int -> [(Int, Int)] -> [ [Int] ]
 
cycle' :: Int -> [(Int, Int)] -> [ [Int] ]
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arrive = fst split
 
arrive = fst split
 
go ls = [ x ++ [snd y] | x <- ls, y <- g, last x == fst y, not (snd y `elem` tail x)]
 
go ls = [ x ++ [snd y] | x <- ls, y <- g, last x == fst y, not (snd y `elem` tail x)]
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  +
</haskell>
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  +
----
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another approach using list as monad
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<haskell>
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cycles :: (Eq a) => a -> [Arc a] -> [[a]]
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cycles _ [] = []
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cycles start arcs =
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map (map fst) $ aux start []
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where
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aux current pathSoFar =
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let nextEdges = filter ((== current) . fst) arcs
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notCyclic = not . (\(_,t) -> (elem t $ map snd pathSoFar))
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noCycles = filter notCyclic nextEdges
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in noCycles >>= \(f,t) -> do
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if (t == start) then return $ pathSoFar ++ (f,t):[(t,t)]
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else aux t (pathSoFar ++ [(f,t)])
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</haskell>
   
 
[[Category:Programming exercise spoilers]]
 
[[Category:Programming exercise spoilers]]

Latest revision as of 06:29, 31 July 2021

Brute-force search from the source, using list comprehension:

 import Data.List (partition)
 cycle' :: Int -> [(Int, Int)] -> [ [Int] ]
 cycle' n g = search [[n]] []
   where search [] result = result
         search cur result = search (go active) (arrive ++ result)
           where split = partition end cur
                 end s = (last s == n) && (length s /= 1)
                 active = snd split
                 arrive = fst split
                 go ls = [ x ++ [snd y] | x <- ls, y <- g, last x == fst y, not (snd y `elem` tail x)]

another approach using list as monad

cycles :: (Eq a) => a -> [Arc a] -> [[a]]
cycles _ [] = []
cycles start arcs = 
    map (map fst) $ aux start []
        where 
            aux current pathSoFar = 
                let nextEdges = filter ((== current) . fst) arcs 
                    notCyclic = not . (\(_,t) -> (elem t $ map snd pathSoFar)) 
                    noCycles  = filter notCyclic nextEdges
                in  noCycles >>= \(f,t) -> do 
                                              if (t == start) then return $ pathSoFar ++ (f,t):[(t,t)]
                                                              else aux t (pathSoFar ++ [(f,t)])