# Difference between revisions of "Euler problems/91 to 100"

## Problem 91

Find the number of right angle triangles in the quadrant.

Solution:

```reduce x y = (quot x d, quot y d)
where d = gcd x y

problem_91 n = 3*n*n + 2* sum others
where
others = do
x1 <- [1..n]
y1 <- [1..n]
let (yi,xi) = reduce x1 y1
let yc = quot (n-y1) yi
let xc = quot x1 xi
return (min xc yc)
```

## Problem 92

Investigating a square digits number chain with a surprising property.

Solution:

```problem_92 = undefined
```

## Problem 93

Using four distinct digits and the rules of arithmetic, find the longest sequence of target numbers.

Solution:

```problem_93 = undefined
```

## Problem 94

Investigating almost equilateral triangles with integral sides and area.

Solution:

```problem_94 = undefined
```

## Problem 95

Find the smallest member of the longest amicable chain with no element exceeding one million.

Solution which avoid visiting a number more than one time :

```import Data.Array.Unboxed
import qualified Data.IntSet as S
import Data.List

takeUntil _ [] = []
takeUntil pred (x:xs) = x : if pred x then takeUntil pred xs else []

chain n s =  lgo [n] \$ properDivisorsSum ! n
where lgo xs x | x > 1000000 || S.notMember x s = (xs,[])
| x `elem` xs = (xs,x : takeUntil (/= x) xs)
| otherwise = lgo (x:xs) \$ properDivisorsSum ! x

properDivisorsSum = accumArray (+) 1 (0,1000000)
\$ (0,-1):[(k,factor)|
factor<-[2..1000000 `div` 2]
, k<-[2*factor,2*factor+factor..1000000]
]

base = S.fromList [1..1000000]

problem_95 = fst \$ until (S.null . snd) f ((0,0),base)
where
f (p@(n,m), s) = (p', s')
where
setMin = head \$ S.toAscList s
(explored, chn) = chain setMin s
len = length chn
p' = if len > m then (minimum chn, len) else p
s' = foldl' (flip S.delete) s explored
```

Here is a more straightforward solution, without optimization. Yet it solves the problem in a few seconds when compiled with GHC 6.6.1 with the -O2 flag. I like to let the compiler do the optimization, without cluttering my code.

This solution avoids using unboxed arrays, which many consider to be somewhat of an imperitive-style hack. In fact, no memoization at all is required.

```import Data.List (foldl1', group)

-- The sum of all proper divisors of n.
d n = product [(p * product g - 1) `div` (p - 1) |
g <- group \$ primeFactors n, let p = head g
] - n

primeFactors = pf primes
where
pf ps@(p:ps') n
| p * p > n = [n]
| r == 0    = p : pf ps q
| otherwise = pf ps' n
where
(q, r) = n `divMod` p

primes = 2 : filter (null . tail . primeFactors) [3,5..]

-- The longest chain of numbers is (n, k), where
-- n is the smallest number in the chain, and k is the length
-- of the chain. We limit the search to chains whose
-- smallest number is no more than m and, optionally, whose
-- largest number is no more than m'.
longestChain m m' = (n, k)
where
(n, Just k) = foldl1' cmpChain [(n, findChain n) | n <- [2..m]]
findChain n = f [] n \$ d n
f s n n'
| n' == n               = Just \$ 1 + length s
| n' < n                = Nothing
| maybe False (< n') m' = Nothing
| n' `elem` s           = Nothing
| otherwise             = f (n' : s) n \$ d n'
cmpChain p@(n, k) q@(n', k')
| (k, negate n) < (k', negate n') = q
| otherwise                       = p

problem_95_v2 = longestChain 1000000 (Just 1000000)
```

## Problem 96

Devise an algorithm for solving Su Doku puzzles.

See numerous solutions on the Sudoku page.

## Problem 97

Find the last ten digits of the non-Mersenne prime: 28433 × 27830457 + 1.

Solution:

```problem_97 = (28433 * 2^7830457 + 1) `mod` (10^10)
```

## Problem 98

Investigating words, and their anagrams, which can represent square numbers.

Solution:

```problem_98 = undefined
```

## Problem 99

Which base/exponent pair in the file has the greatest numerical value?

Solution:

```problem_99 = undefined
```

## Problem 100

Finding the number of blue discs for which there is 50% chance of taking two blue.

Solution:

```problem_100 = undefined
```