Add solutions for problems 17 and 18.

Problem17.hs

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+-- If all the numbers from 1 to 1000 (one thousand) inclusive were written
+-- out in words, how many letters would be used?
+--
+-- NOTE: Do not count spaces or hyphens. For example, 342 (three hundred and
+-- forty-two) contains 23 letters and 115 (one hundred and fifteen) contains
+-- 20 letters. The use of "and" when writing out numbers is in compliance
+-- with British usage.
+
+import Data.Char
+import Data.List
+
+inWords :: Integer -> String
+inWords n
+  | n < 0 = "negative " ++ inWords (negate n)
+  | n <= 19 = [ "zero", "one", "two", "three", "four", "five", "six"
+              , "seven", "eight", "nine", "ten", "eleven", "twelve"
+              , "thirteen", "fourteen", "fifteen", "sixteen"
+              , "seventeen", "eighteen", "nineteen"
+              ] !! fromInteger n
+  | n <= 99 = case n `divMod` 10 of
+      (m, q) -> let tens = [ "twenty", "thirty", "forty", "fifty"
+                           , "sixty", "seventy", "eighty", "ninety" ]
+                in (tens !! fromInteger (m - 2)) ++
+                   (if q == 0 then "" else ("-" ++ inWords q))
+  | n <= 999 = case n `divMod` 100 of
+      (m, 0) -> inWords m ++ " hundred"
+      (m, r) -> inWords m ++ " hundred and " ++ inWords r
+  | n <= 9999 = case n `divMod` 1000 of
+      (m, 0) -> inWords m ++ " thousand"
+      (m, r) | r <= 99   -> inWords m ++ " thousand and " ++ inWords r
+             | otherwise -> inWords m ++ " thousand "     ++ inWords r
+
+main = print $ length $ filter isAlpha $ concatMap inWords [1..1000]

Problem18.hs

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+-- Find the maximum total from top to bottom of the triangle below:
+import Data.List
+
+triangle =
+  [               [ 75 ]
+  ,              [ 95,64 ]
+  ,             [ 17,47,82 ]
+  ,            [ 18,35,87,10 ]
+  ,           [ 20,04,82,47,65 ]
+  ,          [ 19,01,23,75,03,34 ]
+  ,         [ 88,02,77,73,07,63,67 ]
+  ,        [ 99,65,04,28,06,16,70,92 ]
+  ,       [ 41,41,26,56,83,40,80,70,33 ]
+  ,      [ 41,48,72,33,47,32,37,16,94,29 ]
+  ,     [ 53,71,44,65,25,43,91,52,97,51,14 ]
+  ,    [ 70,11,33,28,77,73,17,78,39,68,17,57 ]
+  ,   [ 91,71,52,38,17,14,91,43,58,50,27,29,48 ]
+  ,  [ 63,66,04,68,89,53,67,30,73,16,69,87,40,31 ]
+  , [ 04,62,98,27,23,09,70,98,73,93,38,53,60,04,23 ]
+  ]
+
+maxLR xs = zipWith max xs (tail xs)
+routes   = head . foldr1 (\xs ys -> zipWith (+) xs $ maxLR ys)
+main     = print $ routes triangle