Coding interviews
Reveal answer and reasoning
A lexicographic comparator should compare one field at a time and return as soon as a nonzero result appears. Reverse a comparison by swapping operands rather than negating an arbitrary result. A final unique ID removes ambiguous ties, while null placement and case rules remain explicit parts of the ordered-key contract.
The explicit law harness takes O(k cubed) time for k selected values and constant auxiliary state, which is appropriate for a small diagnostic fixture rather than a sorting algorithm. Passing these finite tests is evidence, not a proof for every custom type or combination of nullable values. Using x minus y, inconsistent null placement or culture-dependent text rules can violate an intended ordering contract.
A comparator must also agree with equality classes expected by the consuming collection. Random examples alone can miss boundary failures, so retain extremes and equal values.
A convincing interview answer treats comparator laws as correctness requirements and uses boundary fixtures to challenge them. It explains the exact one-hundred-fifty check count and why minimum-minus-maximum produces the wrong sign under unchecked arithmetic. It then distinguishes a finite test from a general proof and a static key from a key that changes inside a collection. Senior design discussion should focus on one coherent total order, explicit null semantics and deterministic secondary keys.
Short syntax is useful only after those properties are secure; a subtraction-based one-liner can violate them while looking deceptively simple.
What a strong answer covers
- A lexicographic comparator should compare one field at a time and return as soon as a nonzero result appears.
- The explicit law harness takes O(k cubed) time for k selected values and constant auxiliary state, which is appropriate for a small diagnostic fixture rather than a sorting algorithm.
- Using x minus y, inconsistent null placement or culture-dependent text rules can violate an intended ordering contract.
Common pitfall
Using x minus y, inconsistent null placement or culture-dependent text rules can violate an intended ordering contract. A comparator must also agree with equality classes expected by the consuming collection. Random examples alone can miss boundary failures, so retain extremes and equal values.
Follow-up questions
- Create a nontransitive cyclic order for three named values and make the triple checks fail. Hint: a compares before b, b before c and c before a cannot form an ordinary total order.
- What fails if a key used by an ordered collection mutates after insertion?
Self-review
Choose what to revisit. These marks are your own assessment and are separate from lesson completion.