rhysd

main = print . head $ [ x | x <- [1..], allDivided 20 x ]
    where
        allDivided 1 _ = True
        allDivided n x = x `mod` n == 0 && allDivided (n-1) x

-- もっと真面目に解ける．

rhysd

main = print . sum $ [ x*y | x<-[1..100], y<-[1..100], x/=y ]

rhysd

#include <stdio.h>

int main()
{
    int i,j;
    int result = 0;
    for(i=1; i<=100; ++i){
        for(j=1; j<=100; ++j){
            if(i!=j) result += i*j;
        }

rhysd

main = print $ filter isPrime [2..] !! 10000
  where
    isPrime n = null $ filter (\x->n `mod` x==0) [2..floor $ sqrt $ fromIntegral n]
    -- n が素数かどうかを見るには，[2..sqrt(n)] の数が約数に含まれているかを見れば良い．
        

rhysd

#include <iostream>
#include <string>
#include <numeric>

static const std::string number = "73167176531330624919225119674426574742355349194934"
                                  "96983520312774506326239578318016984801869478851843"
                                  "85861560789112949495459501737958331952853208805511"
                                  "12540698747158523863050715693290963295227443043557"
                                  "66896648950445244523161731856403098711121722383113"
                                  "62229893423380308135336276614282806444486645238749"

rhysd

main = print [a*b*(1000-a-b) | a<-[1..1000], b<-[a..1000], a^2 + b^2 == (1000-a-b)^2]

rhysd

main = print $ sum . filter isPrime $ [2..2000000]
  where
    isPrime n = null $ filter (\x->n `mod` x==0) [2..floor $ sqrt $ fromIntegral n]

-- 142913828922 ( 11.21s with Core i5, 4G Mem )

rhysd

table :: [[Int]]
table = [
        [8,2,22,97,38,15,0,40,0,75,4,5,7,78,52,12,50,77,91,8],
        [49,49,99,40,17,81,18,57,60,87,17,40,98,43,69,48,4,56,62,0],
        [81,49,31,73,55,79,14,29,93,71,40,67,53,88,30,3,49,13,36,65],
        [52,70,95,23,4,60,11,42,69,24,68,56,1,32,56,71,37,2,36,91],
        [22,31,16,71,51,67,63,89,41,92,36,54,22,40,40,28,66,33,13,80],
        [24,47,32,60,99,3,45,2,44,75,33,53,78,36,84,20,35,17,12,50],
        [32,98,81,28,64,23,67,10,26,38,40,67,59,54,70,66,18,38,64,70],
        [67,26,20,68,2,62,12,20,95,63,94,39,63,8,40,91,66,49,94,21],

rhysd

import Data.List

factors :: Int -> [Int]
factors n = filter (\x -> n `mod` x == 0) [1..n]

triNums :: [Int]
triNums = [ sum [1..x] | x<-[1..] ]

main = print $ find (\x -> length(factors x) > 500) triNums

rhysd

import Data.List (maximumBy)
import Data.Function (on)

collatzLength :: Int -> Int
collatzLength 1 = 1
collatzLength n
    | even n = 1 + collatzLength(n `div` 2)
    | odd  n = 1 + collatzLength(3*n + 1)

main = print $ maximumBy (compare `on` collatzLength) [1..1000000]