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SIG (Susquehanna)·Data Scientist·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

SIG (Susquehanna) threw a classic knights-and-knaves logic puzzle at me for a Data Scientist screen. Pretty pure reasoning, no coding, just work through the cases and defend your answer.

Questions Asked (1)

Q1

On an island where Knights always tell the truth and Knaves always lie, person B says to person A: 'You always lie.' Which of the four possible role assignments (Knight/Knight, Knight/Knave, Knave/Knight, Knave/Knave) are logically consistent?

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

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AI HintsAI Generated

Suggested Approach

Systematically test each of the four role assignments by checking whether B's statement about A's truthfulness aligns with the truth-telling or lying behavior dictated by their roles. Eliminate assignments that lead to contradictions, and clearly state which assignments remain logically consistent.

Pro tip: In logic puzzles, it's often efficient to assume a role for one person and see if the other's statement forces a contradiction; also, remember that 'always lies' means every statement they make is false, so if A is a Knave, B's statement 'You always lie' is true, which would require B to be a Knight.

1. Understand the rules

Knights always tell the truth; Knaves always lie. B's statement is 'You always lie,' which is a truth claim about A's truthfulness.

2. Test each assignment

For each of the four role pairs (A, B), determine whether B's statement is true or false based on A's role, then check if that truth value matches B's role (Knight must tell truth, Knave must lie).

3. Identify contradictions

If B's statement truth value does not match B's role, the assignment is inconsistent. Eliminate those assignments.

4. Conclude consistent assignments

List the assignments that survive the consistency check. In this case, only (Knave, Knight) and (Knave, Knave) are consistent.

Key Points to Mention

  • Knights always tell the truth; Knaves always lie.
  • B's statement 'You always lie' is true if and only if A is a Knave.
  • If A is a Knight, B's statement is false, so B must be a Knave.
  • If A is a Knave, B's statement is true, so B must be a Knight.
  • The assignment (Knight, Knight) is inconsistent because B would be telling a falsehood.
  • The assignment (Knight, Knave) is consistent because B lies as required.

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