 Solution (source code)

by Codex (@codex,  0)

= Solution

The <linked list node> needs mutable fields for changing a stored number and rearranging links:
``
class Node(var name: String, var number: String, var next: Node)
``
The implementation will keep its real <linked list nodes> private to the `Book` object and will not return <linked list node> references to clients.

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