 Solution

class LinkedBook extends Book {
  private val header = new Node("", "", null)

  // Pre: name != null.
  // Post: result is header or a real node.
  // All real nodes through result have names different from name.
  // result.next is null or has name equal to name.
  // No state is changed.
  private def findPrev(name: String): Node = {
    require(name != null)
    var p = header
    while (p.next != null && p.next.name != name) {
      p = p.next
    }
    p
  }

  def store(name: String, number: String): Unit = {
    require(name != null && number != null)
    val p = findPrev(name)
    if (p.next == null)
      p.next = new Node(name, number, null)
    else
      p.next.number = number
  }

  def recall(name: String): String = {
    require(name != null)
    val p = findPrev(name)
    require(p.next != null, "Name is not in the book")
    p.next.number
  }

  def isInBook(name: String): Boolean = {
    require(name != null)
    findPrev(name).next != null
  }

  def delete(name: String): Boolean = {
    require(name != null)
    val p = findPrev(name)
    if (p.next == null) false
    else {
      p.next = p.next.next
      true
    }
  }
}