Problem 18

http://projecteuler.net/index.php?section=problems&id=18

上からたどった経路の数字を全て足したときの最大値は? (問題は15段)

  3
 7 5
2 4 6

これだと14が最大値 (3→7→4 or 3→5→6)

import List

main = print $ maximum $ foldl1 f triangle

f xs ys@(_:ys') = zipWith max rs0 rs1
    where rs0 = zipWith (+) xs ys ++ [0]
          rs1 = 0 : zipWith (+) xs ys'

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]
           ]

リストの最後に要素を足すという処理を書きたくないのだが。