What is Project Euler

Project Euler is a website with a collection of programming and mathematical problems. Each challenge varies in difficulty and there are over 600 challenges. I highly recommend anyone interested in puzzles and programming to create an account and start solving. About Project Euler

** Stop here if you have not yet tried and solved Problem 4. **

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Problem 4

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.

Find the largest palindrome made from the product of two 3-digit numbers.

class Problem4
  require 'benchmark'

  def question
    "A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.

    Find the largest palindrome made from the product of two 3-digit numbers."
  end

  def select_largest_palindrome(list = [])
    list = list.sort
    max = nil

    while(max.nil? && !list.empty?) do
      n = list.pop
      max = n if n.to_s == n.to_s.reverse
    end

    max
  end

  def find_largest_palindrome_product
    pairs = (100..999).to_a.product((100..999).to_a)
    products = pairs.map { |pair| pair[0] * pair[1] }

    select_largest_palindrome(products)
  end

  def solve
    find_largest_palindrome_product
  end

  def benchmark
    Benchmark.measure { solve }
  end

  def correct?
    solve === answer
  end

  private

  def answer
    906609
  end
end

 

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