Project Euler Problem 36 Solution

Question

The decimal number, 585 = 1001001001_2 (binary), is palindromic in both bases.

Find the sum of all numbers, less than one million, which are palindromic in base 10 and base 2.

(Please note that the palindromic number, in either base, may not include leading zeros.)

Clojure

#!/usr/bin/env clojure
(defn reversed [string]
  (apply str (reverse string)))

(defn palindrome? [string]
  (= string (reversed string)))

(defn palindrome-in-decimal-and-binary? [n]
  (and (palindrome? (Integer/toString n 2))
       (palindrome? (Integer/toString n))))

(println (reduce + (filter palindrome-in-decimal-and-binary? (range 1 1000000))))

$ time clojure palindromic-bases.clj
real   0m3.059s
user   0m4.238s
sys    0m0.270s

Haskell

palindrome :: Eq a => [a] -> Bool
palindrome s = s == reverse s

digits :: Int -> Int -> [Int]
digits _ 0 = []
digits base n = r : digits base q
    where (q, r) = quotRem n base

main :: IO ()
main = print $ sum [n | n <- [1..1000000], all palindrome [digits 10 n, digits 2 n]]

$ ghc -O2 -o double-palindrome double-palindrome.hs
$ time ./double-palindrome
real   0m0.150s
user   0m0.142s
sys    0m0.008s

Ruby

#!/usr/bin/env ruby
class String
  def palindrome?
    self == self.reverse
  end
end

puts (1..1000000).select { |i| i.to_s.palindrome? && i.to_s(2).palindrome? }.reduce(:+)

$ time ruby binary-palindrome.rb
real   0m0.365s
user   0m0.365s
sys    0m0.000s