diff --git a/2021/day12.hs b/2021/day12.hs
index c605f1e..46f20f6 100644
--- a/2021/day12.hs
+++ b/2021/day12.hs
@@ -1,4 +1,5 @@
 module Day12 where
+import Data.HashSet (HashSet, member, insert, fromList)
 import Data.Map ( Map, fromList, unionWith, empty, (!) )
 import Data.List.Split (splitOn)
 import qualified Data.Char
@@ -15,16 +16,18 @@ addPath (from,to) = unionWith (++) (Data.Map.fromList [(from,[to]), (to,[from])]
 isUpper :: String -> Bool
 isUpper = Data.Char.isUpper . head
 
-pathFinder :: System -> [String] -> [[String]]
-pathFinder sys p@("end":xs) = [p]
-pathFinder sys p@(x:xs) = concatMap (\next -> pathFinder sys (next:p)) elig
-    where elig = filter (\cave -> isUpper cave || cave `notElem` p) (sys ! x)
+pathFinder :: System -> [String] -> HashSet String -> [[String]]
+pathFinder _ [] _ = error "no starting point!"
+pathFinder sys p@("end":xs) _ = [p]
+pathFinder sys p@(x:xs) visited = concatMap (\next -> pathFinder sys (next:p) (next `insert` visited)) elig
+    where elig = filter (\cave -> isUpper cave || not (cave `member` visited)) (sys ! x)
 
-pathFinder2 :: System -> [String] -> [[String]]
-pathFinder2 sys p@("end":xs) = [p]
-pathFinder2 sys p@(x:xs) = concatMap (\next -> f next sys (next:p)) elig
+pathFinder2 :: System -> [String] -> HashSet String -> [[String]]
+pathFinder2 _ [] _ = error "no starting point!"
+pathFinder2 sys p@("end":xs) _ = [p]
+pathFinder2 sys p@(x:xs) visited = concatMap (\next -> f next sys (next:p) (next `insert` visited)) elig
     where
-        f cave = if isUpper cave || cave `notElem` p then pathFinder2 else pathFinder
+        f cave = if isUpper cave || not (cave `member` visited) then pathFinder2 else pathFinder
         elig = filter (/= "start") (sys ! x)
 
 main :: IO ()
@@ -33,9 +36,9 @@ main = do
     let system = foldr addPath Data.Map.empty input
 
     putStrLn "part 1: "
-    let paths = map reverse $ pathFinder system ["start"]
+    let paths = map reverse $ pathFinder system ["start"] (Data.HashSet.fromList ["start"])
     print $ length paths
 
     putStrLn "part 2: "
-    let paths = map reverse $ pathFinder2 system ["start"] 
+    let paths = map reverse $ pathFinder2 system ["start"] (Data.HashSet.fromList ["start"])
     print $ length paths