Cracking Coding Interview - Remove Dups Linked List

def remove_dups(head_node)
    unique_values = {}
    current_node = head_node
    prev_node = nil
    while current_node.next != nil
        if unique_values.key?(current_node.value)
            prev_node.next = current_node.next
        else
            unique_values[current_node.value] = true
            prev_node = current_node
        end
        current_node = current_node.next
    end
    current_node
end