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Euler problems/101 to 110

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Solution:
Solution:
<haskell>
<haskell>
-
problem_102 = undefined
+
import List
 +
 
 +
split :: Char -> String -> [String]
 +
split = unfoldr . split'
 +
 
 +
split' :: Char -> String -> Maybe (String, String)
 +
split' c l
 +
| null l = Nothing
 +
| otherwise = Just (h, drop 1 t)
 +
where (h, t) = span (/=c) l
 +
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)
 +
sToInt x=map ((+0).read) $split ',' x
 +
problem_102=do
 +
x<-readFile "triangles.txt"
 +
print $length$ filter isOrig$map sToInt $lines x
</haskell>
</haskell>

Revision as of 12:50, 9 December 2007

Contents

1 Problem 101

Investigate the optimum polynomial function to model the first k terms of a given sequence.

Solution: 

problem_101 = undefined

2 Problem 102

For how many triangles in the text file does the interior contain the origin?

Solution: 

import List
 
split :: Char -> String -> [String]
split = unfoldr . split'
 
split' :: Char -> String -> Maybe (String, String)
split' c l
  | null l = Nothing
  | otherwise = Just (h, drop 1 t)
  where (h, t) = span (/=c) l
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)
sToInt x=map ((+0).read) $split ',' x
problem_102=do
    x<-readFile "triangles.txt"
    print $length$ filter isOrig$map sToInt $lines x

3 Problem 103

Investigating sets with a special subset sum property.

Solution: 

problem_103 = undefined

4 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 

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

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.

5 Problem 105

Find the sum of the special sum sets in the file.

Solution: 

problem_105 = undefined

6 Problem 106

Find the minimum number of comparisons needed to identify special sum sets.

Solution: 

problem_106 = undefined

7 Problem 107

Determining the most efficient way to connect the network.

Solution: 

problem_107 = undefined

8 Problem 108

Solving the Diophantine equation 1/x + 1/y = 1/n.

Solution: 

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

9 Problem 109

How many distinct ways can a player checkout in the game of darts with a score of less than 100?

Solution: 

problem_109 = undefined

10 Problem 110

Find an efficient algorithm to analyse the number of solutions of the equation 1/x + 1/y = 1/n.

Solution: 

-- prob in problem_108
problem_110 = prob 13 (8*10^6)
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