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== [http://projecteuler.net/index.php?section=problems&id=101 Problem 101] ==
+
Do them on your own!
Investigate the optimum polynomial function to model the first k terms of a given sequence.
 
 
Solution:
 
<haskell>
 
problem_101 = undefined
 
</haskell>
 
 
== [http://projecteuler.net/index.php?section=problems&id=102 Problem 102] ==
 
For how many triangles in the text file does the interior contain the origin?
 
 
Solution:
 
<haskell>
 
import Text.Regex
 
--ghc -M p102.hs
 
isOrig (x1:y1:x2:y2:x3:y3:[])=
 
t1*t2>=0 && t3*t4>=0 && t5*t6>=0
 
where
 
x4=0
 
y4=0
 
t1=(y2-y1)*(x4-x1)+(x1-x2)*(y4-y1)
 
t2=(y2-y1)*(x3-x1)+(x1-x2)*(y3-y1)
 
t3=(y3-y1)*(x4-x1)+(x1-x3)*(y4-y1)
 
t4=(y3-y1)*(x2-x1)+(x1-x3)*(y2-y1)
 
t5=(y3-y2)*(x4-x2)+(x2-x3)*(y4-y2)
 
t6=(y3-y2)*(x1-x2)+(x2-x3)*(y1-y2)
 
buildTriangle s = map read (splitRegex (mkRegex ",") s) :: [Integer]
 
problem_102=do
 
x<-readFile "triangles.txt"
 
let y=map buildTriangle$lines x
 
print $length$ filter isOrig y
 
</haskell>
 
 
== [http://projecteuler.net/index.php?section=problems&id=103 Problem 103] ==
 
Investigating sets with a special subset sum property.
 
 
Solution:
 
<haskell>
 
six=[11,18,19,20,22,25]
 
seven=[mid+a|let mid=six!!3,a<-0:six]
 
problem_103=foldl (++) "" $map show seven
 
</haskell>
 
 
== [http://projecteuler.net/index.php?section=problems&id=104 Problem 104] ==
 
Finding Fibonacci numbers for which the first and last nine digits are pandigital.
 
 
Solution:
 
 
Very nice problem. I didnt realize you could deal with the precision problem.
 
Therefore I used this identity to speed up the fibonacci calculation:
 
f_(2*n+k)
 
= f_k*(f_(n+1))^2
 
+ 2*f_(k-1)*f_(n+1)*f_n
 
+ f_(k-2)*(f_n)^2
 
 
<haskell>
 
import Data.List
 
import Data.Char
 
 
fibos = rec 0 1
 
where
 
rec a b = a:rec b (a+b)
 
 
fibo_2nk n k =
 
let
 
fkm1 = fibo (k-1)
 
fkm2 = fibo (k-2)
 
fk = fkm1 + fkm2
 
fnp1 = fibo (n+1)
 
fnp1sq = fnp1^2
 
fn = fibo n
 
fnsq = fn^2
 
in
 
fk*fnp1sq + 2*fkm1*fnp1*fn + fkm2*fnsq
 
 
fibo x =
 
let
 
threshold = 30000
 
n = div x 3
 
k = n+mod x 3
 
in
 
if x < threshold
 
then fibos !! x
 
else fibo_2nk n k
 
 
findCandidates = rec 0 1 0
 
where
 
m = 10^9
 
rec a b n =
 
let
 
continue = rec b (mod (a+b) m) (n+1)
 
isBackPan a = (sort $ show a) == "123456789"
 
in
 
if isBackPan a
 
then n:continue
 
else continue
 
search =
 
let
 
isFrontPan x = (sort $ take 9 $ show x) == "123456789"
 
in
 
map fst
 
$ take 1
 
$ dropWhile (not.snd)
 
$ zip findCandidates
 
$ map (isFrontPan.fibo) findCandidates
 
 
problem_104 = search
 
</haskell>
 
It took 8 sec on a 2.2Ghz machine.
 
 
The lesson I learned fom this challenge, is: know mathematical identities and exploit them. They allow you take short cuts.
 
Normally you compute all previous fibonacci numbers to compute a random fibonacci number. Which has linear costs. The aforementioned identity builds the number not from its two predecessors but from 4 much smaller ones. This makes the algorithm logarithmic in its complexity. It really shines if you want to compute a random very large fibonacci number. f.i. the 10mio.th fibonacci number which is over 2mio characters long, took 20sec to compute on my 2.2ghz laptop.
 
== [http://projecteuler.net/index.php?section=problems&id=105 Problem 105] ==
 
Find the sum of the special sum sets in the file.
 
 
Solution:
 
<haskell>
 
import Data.List
 
import Control.Monad
 
import Text.Regex
 
 
solNum=map solve [7..12]
 
solve n = twoSetsOf [0..n-1] =<< [2..div n 2]
 
twoSetsOf xs n = do
 
firstSet <- setsOf n xs
 
let rest = dropWhile (/= head firstSet) xs \\ firstSet
 
secondSet <- setsOf n rest
 
let f = firstSet >>= enumFromTo 1
 
s = secondSet >>= enumFromTo 1
 
guard $ not $ null (f \\ s) || null (s \\ f)
 
[(firstSet,secondSet)]
 
 
setsOf 0 _ = [[]]
 
setsOf (n+1) xs = concat [map (y:) (setsOf n ys) | (y:ys) <- tails xs]
 
comp lst a b=
 
a1/=b1
 
where
 
a1=sum$map (lst!!) a
 
b1=sum$map (lst!!) b
 
notEqu lst =
 
all id[comp slst a b|(a,b)<-solNum!!s]
 
where
 
s=length lst-7
 
slst=sort lst
 
moreElem lst =
 
all id maE
 
where
 
le=length lst
 
sortLst=sort lst
 
maxElem =
 
(-1):[sum $drop (le-a) sortLst|
 
a<-[0..le]
 
]
 
minElem =
 
[sum $take a sortLst|
 
a<-[0..le]
 
]
 
maE=[a<b|(a,b)<-zip maxElem minElem]
 
stoInt s=map read (splitRegex (mkRegex ",") s) :: [Integer]
 
check x=moreElem x && notEqu x
 
main = do
 
f <- readFile "sets.txt"
 
let sets = map stoInt$ lines f
 
let ssets = filter check sets
 
print $ sum $ concat ssets
 
</haskell>
 
 
== [http://projecteuler.net/index.php?section=problems&id=106 Problem 106] ==
 
Find the minimum number of comparisons needed to identify special sum sets.
 
 
Solution:
 
<haskell>
 
binomial x y =div (prodxy (y+1) x) (prodxy 1 (x-y))
 
prodxy x y=product[x..y]
 
-- http://mathworld.wolfram.com/DyckPath.html
 
catalan n=flip div (n+1) $binomial (2*n) n
 
calc n=
 
sum[e*(c-d)|
 
a<-[1..di2],
 
let mu2=a*2,
 
let c=flip div 2 $ binomial mu2 a,
 
let d=catalan a,
 
let e=binomial n mu2]
 
where
 
di2=div n 2
 
problem_106 = calc 12
 
</haskell>
 
 
== [http://projecteuler.net/index.php?section=problems&id=107 Problem 107] ==
 
Determining the most efficient way to connect the network.
 
 
Solution:
 
<haskell>
 
import Control.Monad.ST
 
import Control.Monad
 
import Data.Array.MArray
 
import Data.Array.ST
 
import Data.List
 
import Data.Map (fromList,(!))
 
import Text.Regex
 
makeArr x=map zero (splitRegex (mkRegex ",") x)
 
makeNet x lst y=[((a,b),m)|a<-[0..x-1],b<-[0..a-1],let m=lst!!a!!b,m/=y]
 
zero x
 
|'-' `elem` x=0
 
|otherwise=read x::Int
 
problem_107 =do
 
a<-readFile "network.txt"
 
let b=map makeArr $lines a
 
network = makeNet 40 b 0
 
edges = sortBy (\x y->compare (snd x) (snd y)) network
 
eedges =map fst edges
 
mape=fromList edges
 
d=sum $ map snd edges
 
e=sum$map (mape!)$kruskal eedges
 
print (d-e)
 
kruskal es = runST ( do
 
let hi = maximum $ map (uncurry max) es
 
lo = minimum $ map (uncurry min) es
 
djs <- makeDjs (lo,hi)
 
filterM (kruskalST djs) es)
 
 
kruskalST djs (u,v) = do
 
disjoint <- djsDisjoint u v djs
 
when disjoint $ djsUnion u v djs
 
return disjoint
 
 
type DisjointSet s = STArray s Int (Maybe Int)
 
 
makeDjs :: (Int,Int) -> ST s (DisjointSet s)
 
makeDjs b = newArray b Nothing
 
 
djsUnion a b djs = do
 
root <- djsFind a djs
 
writeArray djs root $ Just b
 
 
djsFind a djs = maybe (return a) f =<< readArray djs a
 
where f p = do p' <- djsFind p djs
 
writeArray djs a (Just p')
 
return p'
 
 
djsDisjoint a b uf = liftM2 (/=) (djsFind a uf) (djsFind b uf)
 
</haskell>
 
 
== [http://projecteuler.net/index.php?section=problems&id=108 Problem 108] ==
 
Solving the Diophantine equation 1/x + 1/y = 1/n.
 
 
Solution:
 
<haskell>
 
import List
 
primes=[2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73]
 
series _ 1 =[[0]]
 
series xs n =[x:ps|x<-xs,ps<-series [0..x] (n-1) ]
 
distinct=product. map (+1)
 
sumpri x=product $map (\(x,y)->x^y)$zip primes x
 
prob x y =head$sort[(sumpri m ,m)|m<-series [1..3] x,(>y)$distinct$map (*2) m]
 
problem_108=prob 7 2000
 
</haskell>
 
 
== [http://projecteuler.net/index.php?section=problems&id=109 Problem 109] ==
 
How many distinct ways can a player checkout in the game of darts with a score of less than 100?
 
 
Solution:
 
<haskell>
 
problem_109 = undefined
 
</haskell>
 
 
== [http://projecteuler.net/index.php?section=problems&id=110 Problem 110] ==
 
Find an efficient algorithm to analyse the number of solutions of the equation 1/x + 1/y = 1/n.
 
 
Solution:
 
<haskell>
 
-- prob in problem_108
 
problem_110 = prob 13 (8*10^6)
 
</haskell>
 

Revision as of 21:45, 29 January 2008

Do them on your own!