## Question

Working from left-to-right if no digit is exceeded by the digit to its left it is called an increasing number; for example, 134468.

Similarly if no digit is exceeded by the digit to its right it is called a decreasing number; for example, 66420.

We shall call a positive integer that is neither increasing nor decreasing a “bouncy” number; for example, 155349.

Clearly there cannot be any bouncy numbers below one-hundred, but just over half of the numbers below one-thousand (525) are bouncy. In fact, the least number for which the proportion of bouncy numbers first reaches 50% is 538.

Surprisingly, bouncy numbers become more and more common and by the time we reach 21780 the proportion of bouncy numbers is equal to 90%.

Find the least number for which the proportion of bouncy numbers is exactly 99%.

import Data.List (sort, find)

isIncreasing ::  Integer -> Bool
isIncreasing n = show n == sort (show n)

isDecreasing ::  Integer -> Bool
isDecreasing n = show n == reverse (sort (show n))

isBouncy ::  Integer -> Bool
isBouncy n = not (isIncreasing n) && not (isDecreasing n)

bouncy ::  [(Integer, Integer)]
bouncy = b' 1 0 where
b' n acc | isBouncy n = (n, acc+1) : b' (n+1) (acc+1)
| otherwise  = (n, acc)   : b' (n+1) acc

main ::  IO ()
main = print $maybe 0 fst$ find (\(n, b) -> (n * 99) == (b * 100)) bouncy
$ghc -O2 -o bouncy bouncy.hs$ time ./bouncy
real   0m1.140s
user   0m1.128s
sys    0m0.000s

## Python

#!/usr/bin/env python
from itertools import *

def pairwise(iterable):
"s -> (s0,s1), (s1,s2), (s2, s3), ..."
a, b = tee(iterable)
next(b, None)
return zip(a, b)

def digits(n):
return list(map(int, str(n)))

def is_increasing(n):
return all(prev <= curr for prev, curr in pairwise(digits(n)))

def is_decreasing(n):
return all(prev >= curr for prev, curr in pairwise(digits(n)))

def is_bouncy(n):
return not is_increasing(n) and not is_decreasing(n)

def running_total(iterable):
total = 0
for element in iterable:
total += element
yield total

def main():
nums = count(1)
bouncy = running_total(map(lambda n: float(is_bouncy(n)), count(1)))
print(next((n for n, b in zip(nums, bouncy) if b / n == 0.99)))

if __name__ == "__main__": main()
\$ time python3 bouncy.py
real   0m17.846s
user   0m17.720s
sys    0m0.008s

Questions? Comments? Send me an email: z@chdenton.com