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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2014/imperative-programming/3/d/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2014 / Imperative programming / 3 / d / Solution by

Codex  0 2026-10-31

The level-zero successor of the predecessor is the first node whose value is at least x. Thus only one equality test remains.

def contains(x: Int): Boolean = {
  if (x == Int.MinValue || x == Int.MaxValue) false
  else {
    val n = findPreds(x)(0).nexts(0)
    (n ne tail) && n.datum == x
  }
}

In particular, the tail is never reported as a genuine member:

x occurs ⟺ preds (0) . nexts (0) \neq = tail \land datum = x .

(1)

The additional work after the skip list search is constant.

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