# you can write to stdout for debugging purposes, e.g.
# puts "this is a debug message"
def solution(a)
    # write your code in Ruby 2.2
    length = slow = fast = 0
    loop do
        fast = a[a[fast]]
        slow = a[slow]
        if fast == slow
          loop do
            fast = a[a[fast]]
            slow = a[slow]
            length += 1
            if fast == slow
                return length
            end
          end
        end
    end
end