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Warner Bros Discovery·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Got a coding question at Warner Bros Discovery that looked straightforward on the surface but had a subtle twist baked in. The in-place constraint is what makes it interesting.

Questions Asked (1)

Q1

Given an array, rotate it left by k positions and then rotate it back right by k positions. Both operations must be done in-place using O(1) extra space.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

The left-then-right thing felt redundant at first and I almost said so out loud.

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AI HintsAI Generated

Suggested Approach

First, clarify that rotating left by k and then right by k returns the array to its original state, so the net effect is no change. Then, explain the in-place reversal algorithm for each rotation: to rotate left by k, reverse the first k elements, reverse the remaining n-k elements, and finally reverse the entire array; to rotate right by k, reverse the entire array, then reverse the first k elements, and then reverse the remaining n-k elements. Emphasize that both operations use O(1) extra space and that performing them sequentially restores the original array.

Pro tip: Mention that if the goal is to actually perform both rotations, you can optimize by recognizing they cancel out, but if the interviewer wants to see the implementation, demonstrate the reversal method clearly and note that the second rotation is the inverse of the first.

1. Clarify the problem and constraints

Confirm that the array should be rotated left by k, then right by k, and that both operations must be in-place with O(1) extra space. Ask if k can be larger than the array length and how to handle that (e.g., k = k % n).

2. Explain the reversal algorithm for left rotation

Describe how to rotate left by k in-place: reverse the first k elements, reverse the remaining n-k elements, then reverse the entire array. This uses O(1) extra space and runs in O(n) time.

3. Explain the reversal algorithm for right rotation

Describe how to rotate right by k in-place: reverse the entire array, then reverse the first k elements, then reverse the remaining n-k elements. Again, O(1) extra space and O(n) time.

4. Analyze the combined effect

Show that performing left rotation by k followed by right rotation by k returns the array to its original state. If the interviewer expects the array to be modified, clarify that the net result is no change; otherwise, implement both rotations sequentially.

5. Discuss time and space complexity

State that each rotation takes O(n) time and O(1) extra space, so the combined operation also takes O(n) time and O(1) extra space. Mention that if the operations cancel, you could simply return the original array without any modifications.

Key Points to Mention

  • In-place rotation using reversal algorithm
  • Time complexity O(n) and space complexity O(1)
  • Handling k larger than array length with modulo operation
  • The net effect of left rotation by k followed by right rotation by k is the identity transformation
  • Edge cases: empty array, k=0, k=n
  • Trade-offs: simplicity of reversal vs. other methods like juggling algorithm

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