Ruby find dominator/leader element

dominator.rb

class Dominator
    def solution(a)
        counter = 0
        candidate = 0

        a.each do |elem|
            if counter == 0
                candidate = elem
                counter += 1
            elsif candidate == elem
                counter += 1
            else
                counter -= 1
            end
        end

        occurence = a.inject(0) { |sum, x| sum + if x == candidate then 1 else 0 end }
        if occurence > (a.length / 2)
            a.find_index(candidate)
        else
            -1
        end
    end
end


puts Dominator.new.solution([3, 0, 1, 1, 4, 1, 1])