solve problem 4.

1 files changed, 39 insertions, 0 deletions

diff --git a/spec/euler/problem_four_spec.rb b/spec/euler/problem_four_spec.rb
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--- /dev/null
+++ b/spec/euler/problem_four_spec.rb
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+#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.
+describe 'problem four' do
+  class Palindrome
+    def largest(digits:1)
+      min = ([1]  + ([0] * (digits - 1))).flatten.join('').to_i
+      max = ([9] * digits).flatten.join('').to_i
+      puts [min, max].inspect
+
+      max.downto(min) do |m|
+        max.downto(min) do |n|
+          result = m * n
+          return result if palindrome?(result)
+        end
+      end
+    end
+
+    private
+
+    def palindrome?(number)
+      number.to_s == number.to_s.reverse
+    end
+  end
+
+  subject { Palindrome.new }
+
+  it 'can find the largest palindrom that is the product of two one digit numbers' do
+    expect(subject.largest(digits: 1)).to eql(9)
+  end
+
+  it 'can find the largest palindrom that is the product of two digits' do
+    expect(subject.largest(digits: 2)).to eql(9009)
+  end
+
+  it 'can find the largest palindrome that is the product of two three digit numbers' do
+    expect(subject.largest(digits: 3)).to eql(580085)
+  end
+end