Add solution for the Project Euler Problem 4

Project Euler/Problem 4/problem4_sol1.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+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