Project-Euler
Ruby

# Largest Palindrome Product

# 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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