# Euler problems/101 to 110

## Contents

## Problem 101

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

Solution:

```
problem_101 = undefined
```

## 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
```

## Problem 103

Investigating sets with a special subset sum property.

Solution:

```
problem_103 = undefined
```

## 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.

## Problem 105

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

Solution:

```
problem_105 = undefined
```

## Problem 106

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

Solution:

```
problem_106 = undefined
```

## Problem 107

Determining the most efficient way to connect the network.

Solution:

```
problem_107 = undefined
```

## 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
```

## 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
```

## 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)
```