Haskell Notes

Haskell Tape Equilibrium

tapeEquilibrium :: [Int] -> Int
tapeEquilibrium a = result
  where
    (_, _, result) = foldl step (0, sum a, maxBound) [0 .. length a - 2]

    step (firstPart, secondPart, best) i =
      let firstPart'  = firstPart + i
          secondPart' = secondPart - i
          diff        = abs (firstPart' - secondPart')
      in  (firstPart', secondPart', min best diff)

This keeps left and right running sums and updates the smallest difference at each split point.

Haskell Triangle

import Data.List (sort)

triangle :: [Int] -> Int
triangle a
  | length s < 3                       = 0
  | any valid (zip3 s (drop 1 s) (drop 2 s)) = 1
  | otherwise                           = 0
  where
    s = sort a
    valid (x, y, z) = x > 0 && x > (z - y)

This sorts the values and checks nearby triples, because a valid triangle only needs one local match after sorting.