haskell: add [p12, p17]

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

haskell/p17.hs

@@ -0,0 +1,22 @@
+--The answer is written for numbers <= 1000
+
+main :: IO ()
+main = putStr $ show $ solve 1000
+
+solve :: Int -> Int
+solve n = sum $ map (length . numWord) [1 .. n]
+
+numWord :: Int -> [Char]
+numWord n
+  | n == 0 = ""
+  | n == 1000 = "onethousand"
+  | n < 20 = ones !! (n - 1)
+  | n < 100 = tens !! (div n 10 - 2) ++ numWord (mod n 10)
+  | mod n 100 == 0 = numWord (div n 100) ++ "hundred"
+  | otherwise = numWord (div n 100) ++ "hundredand" ++ numWord (mod n 100)
+  where
+    tens :: [[Char]]
+    tens = ["twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"]
+
+    ones :: [[Char]]
+    ones = ["one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"]