Project Euler欧拉计划

1

answer = sum $ takeWhile (< 1000) [ n | n <- [ 1 .. 999 ],
mod n 3 == 0 || mod n 5 == 0
]

2

answer = sum $ filter even $ takeWhile (<= 4000000) $ map fst $
iterate (\ (a,b) -> (b, a+b))
(1, 2)

3

answer = maximum $ fact 600851475143
where
-- Get the factors of N, not sorted
fact n = factors n 2
where
-- Get the factors of N
-- p <= (the next factor of n),  p <- Z+
factors n p | n == 1     = [1]
| n  > 1     = if    mod n p == 0              -- if    p is a factor
then  p:(factors (div n p) p)
else  factors n (succ p)        -- else  try next

4

answer = maximum $ [ x*y | x <- [ 100 .. 999 ],
y <- [ 100 .. 999 ],
isPalindrome $ show(x*y)
]
where
isPalindrome l = l == reverse(l)

5

foldr lcm 1 [1..20]

6

ans = s2 - s1
where s1 = sum $ map (\ a -> a^2 ) [ 1 .. 100 ]
s2 = (sum [ 1 .. 100 ])^2

8

import Data.Char
ans = maximum productLst
where productLst = getProductLst number
where number = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"
getProductLst l | len == 13   = [ productOf1to13 ]
| otherwise   = productOf1to13:getProductLst(tail l)
where len = length l
productOf1to13 = product from1to13
where from1to13 = map (\ a -> read (a:[]) :: Integer) $ take 13 l
--                                                                 ^^^^^^^
--                                           Necessary. Or you will get some negative numbers.

9

ans = product' $ head $ [ (a, b, c) | c <- [ 1 .. 1000 ],
b <- [ 1 .. c ],
a <- [ 1 .. b ],
a + b + c == 1000,
a^2 + b^2 == c^2
]
where product' (x, y, z) = x * y * z