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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
results = []
max.downto(min) do |m|
max.downto(min) do |n|
result = m * n
results.push(result) if palindrome?(result)
end
end
results.max
end
private
def palindrome?(number)
number.to_s == number.to_s.reverse
end
end
subject { Palindrome.new }
it 'can find the largest palindrome that is the product of two one digit numbers' do
expect(subject.largest(digits: 1)).to eql(9)
end
it 'can find the largest palindrome 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(906609)
end
end
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