# Definition for singly-linked list.
# class ListNode
#     attr_accessor :val, :next
#     def initialize(val)
#         @val = val
#         @next = nil
#     end
# end

# @param {ListNode} head
# @return {ListNode}

require 'set'

def detectCycle(head)
    def detectCycleRec(node, seen)
        if node == nil then
            return node
        elsif seen.add?(node) == nil then
            return node
        end

        return detectCycleRec(node.next, seen)
    end

    detectCycleRec(head, Set.new)
end