Merge pull request #56 from vzvu3k6k/project-euler-4

Add solution for the Project Euler Problem 4

DIRECTORY.md

@@ -43,6 +43,7 @@
     * [Sol2](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_3/sol2.rb)
   * Problem 4
     * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_4/sol1.rb)
+    * [Sol2](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_4/sol2.rb)
   * Problem 5
     * [Sol1](https://github.com/TheAlgorithms/Ruby/blob/master/project_euler/problem_5/sol1.rb)
 

project_euler/problem_4/sol2.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+# 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 Integer
+  def parindrome?
+    self == reverse
+  end
+
+  # 123.reverse == 321
+  # 100.reverse == 1
+  def reverse
+    result = 0
+    n = self
+    loop do
+      result = result * 10 + n % 10
+      break if (n /= 10).zero?
+    end
+    result
+  end
+end
+
+factors = (100..999).to_a
+products = factors.product(factors).map { _1 * _2 }
+puts products.select(&:parindrome?).max