Anthropic·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Coding screen for a software engineer role at Anthropic. One problem, simulation-style, and I burned way too much time just parsing what the question was even asking. Got to a working solution but the clock ran out before any follow-ups.

Questions Asked (1)

You're given a list of instructions (plus, next, jump) that operate on a global accumulator and a program counter. Exactly one line has its 'jump' and 'next' instructions swapped, causing the program to loop infinitely. Find that line, fix it, and return the accumulated value once the program terminates without looping.

Algorithms & Data StructuresRoot Cause Analysis

Author's notes

Spent an embarrassing chunk of time convinced the entire instruction set was broken, not just one line.

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AI HintsAI Generated

Suggested Approach

Simulate the program while tracking visited instruction indices to detect the infinite loop. Once a cycle is detected, identify the instruction where swapping 'jump' and 'next' would break the cycle, then re-run the corrected program to compute the final accumulator value.

Pro tip: Before coding, clarify edge cases like negative jumps, out-of-bounds jumps, and whether the swap is guaranteed to exist; this shows thoroughness and prevents wasted effort.

1. Simulate and detect loop

Execute the program step-by-step, maintaining a set of visited instruction indices. When an index is revisited, you've found the infinite loop.

2. Identify candidate swap

The swapped line must be one of the instructions executed in the loop. For each such instruction, consider swapping its operation and test if it breaks the loop.

3. Test swap and compute result

For each candidate, simulate the modified program from the start. If it terminates without looping, record the accumulator value.

4. Return final accumulator

Once the correct swap is found and the program terminates, return the accumulated value.

Key Points to Mention

Use a visited set to detect cycles efficiently.
Only instructions within the loop are candidates for the swap.
Swapping 'jump' and 'next' changes control flow, not the accumulator directly.
Simulation must handle both positive and negative jumps.
Time complexity: O(n) for initial simulation, O(k*n) for testing candidates (k = loop size).
Edge cases: jump to out-of-bounds, immediate termination, multiple loops.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.