diff --git a/3/3.hs b/3/3.hs
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+{-# LANGUAGE Strict #-}
+module Main where
+
+import Control.Applicative (some)
+import Data.Bits (Bits(shift))
+import qualified Data.Tree as T
+import Data.List (maximumBy, foldl', nub)
+import Data.Ord (comparing)
+import Text.Parsec hiding (Empty)
+
+import Data.Maybe (catMaybes)
+
+data Rect = Rect { rectID     :: Int
+                 , rectLeft   :: Int
+                 , rectTop    :: Int
+                 , rectWidth  :: Int
+                 , rectHeight :: Int
+                 } deriving (Eq)
+
+instance Show Rect where
+  show (Rect id l t w h) = "#" ++ show id ++ " " ++ show l ++ "," ++ show t ++ ":" ++ show (l+w) ++ "," ++ show (t+h)
+
+inputP :: Parsec String () Rect
+inputP =
+  (\id (l,t) (w,h) -> Rect id l t w h)
+  <$> (idP <* spaces <* char '@' <* spaces)
+  <*> (posP <* char ':' <* spaces)
+  <*> dimP
+  where
+    intP = read <$> some digit
+    idP = char '#' *> intP
+    posP = (,) <$> (intP <* char ',') <*> intP
+    dimP = (,) <$> (intP <* char 'x') <*> intP
+
+readInput :: [String] -> [Rect]
+readInput ls = case traverse (parse inputP "") ls of
+  Left e -> error (show e)
+  Right rs -> rs
+
+sheetSize :: [Rect] -> (Int, Int)
+sheetSize rects =
+  ( calcBound (\(Rect _ l _ w _) -> l + w)
+  , calcBound (\(Rect _ _ t _ h) -> t + h))
+  where
+    calcBound f = f (maximumBy (comparing f) rects)
+
+isOverlapCell rects cell =
+  (> 1) . length . take 2 . filter (cellInRect cell) $ rects
+  where
+    cellInRect (x, y) (Rect _ l t w h) =
+      l <= x && (l+w) >= (x+1) && t <= y && (t+h) >= (y+1)
+
+---------------- Brute force ----------------
+
+bruteForceSolve rects =
+  let (w, h) = sheetSize rects
+      cells = [(i, j) | i <- [0..w-1], j <- [0..h-1]]
+      overlapArea = length . filter (isOverlapCell rects) $ cells
+  in overlapArea
+
+---------------- Interval tree ----------------
+
+newtype Interval a = Interval (a,a) deriving (Eq)
+
+instance Show a => Show (Interval a) where
+  show (Interval (a, b)) = "<" ++ show a ++ "," ++ show b ++ ">"
+
+data IntervalTree a b = Node { itLeft :: IntervalTree a b
+                             , itCenter :: a
+                             , itIntervals :: [(Interval a, b)]
+                             , itRight :: IntervalTree a b
+                             }
+                      | Empty a a deriving (Show, Eq)
+
+insert :: (Ord a, Bits a, Num a) => (Interval a, b) -> IntervalTree a b -> IntervalTree a b
+insert o@(interval, _) tree = case tree of
+  Empty start end -> go start end (start + half (end - start))
+  Node l center is r | interval `leftOf` center   -> Node (insert o l) center is r
+  Node l center is r | interval `rightOf` center  -> Node l center is (insert o r)
+  Node l center is r -> Node l center (o:is) r
+  where
+    go start end center
+      | interval `leftOf` center   = Node (insert o (Empty start center)) center [] (Empty center end)
+      | interval `rightOf` center  = Node (Empty start center) center [] (insert o (Empty center end))
+      | otherwise                  = Node (Empty start center) center [o] (Empty center end)
+
+    rightOf (Interval (start, _)) x = x < start
+    leftOf (Interval (_, end)) x = end <= x
+    half = flip shift (-1)
+
+includingIntervals :: Ord a => Interval a -> IntervalTree a b -> [(Interval a, b)]
+includingIntervals interval = go []
+  where
+    go acc t = case t of
+      Empty _ _ -> acc
+      Node l _ is r -> go (go (filter (\(i,_) -> i `includes` interval) is ++ acc) l) r
+
+    includes (Interval (start1, end1)) (Interval (start2, end2))
+      = start1 <= start2 && end2 <= end1
+
+fromList :: (Ord a, Bits a, Num a) => a -> a -> [(Interval a, b)] -> IntervalTree a b
+fromList start end = foldl' (flip insert) (Empty start end)
+
+rectIntervalTrees :: [Rect] -> (IntervalTree Int Rect, IntervalTree Int Rect)
+rectIntervalTrees rects =
+  let (w, h) = sheetSize rects
+  in ( fromList 0 w . map (\r -> (toInterval rectLeft rectWidth r, r)) $ rects
+     , fromList 0 h . map (\r -> (toInterval rectTop rectHeight r, r)) $ rects
+     )
+  where
+    toInterval pos dim rect = Interval (pos rect, pos rect + dim rect)
+
+toTree :: (Show a, Show b) => IntervalTree a b -> T.Tree String
+toTree (Empty start end) = T.Node (show ("E", start, end)) []
+toTree (Node l c is r) = T.Node (show ("N", c, is)) [toTree l, toTree r]
+
+intervalTreeSolve :: [Rect] -> Int
+intervalTreeSolve rects =
+  let (w, h) = sheetSize rects
+      cells = [(i, j) | i <- [0..w-1], j <- [0..h-1]]
+      (xTree, yTree) = rectIntervalTrees rects
+      overlapArea = length . filter (\c -> isOverlapCell (cellRects xTree yTree c) c) $ cells
+  in overlapArea
+  where      
+    cellRects xTree yTree (x,y) = 
+      nub . map snd
+      $ includingIntervals (Interval (x, x+1)) xTree ++ includingIntervals (Interval (y, y+1)) yTree
+
+main :: IO ()
+main = do
+  rects <- readInput . lines <$> getContents
+  let solution = intervalTreeSolve rects
+  putStrLn $ "Overlap Area = " ++ show solution
\ No newline at end of file