Two Sum with Hash Map
As a Engineering Manager engineer, explain Two Sum with Hash Map. What problem does it solve and how would you describe it in an interview?
Answers use simple, clear English.
Quick interview answer
At Engineering Manager depth: start with the problem, then mechanism, then a short example. Find two indices whose values sum to target in one pass by storing value→index in a hash map and looking up complement = target − current on each step.
Detailed answer
At Engineering Manager depth: start with the problem, then mechanism, then a short example. Find two indices whose values sum to target in one pass by storing value→index in a hash map and looking up complement = target − current on each step. Core: Find two indices whose values sum to target in one pass by storing value→index in a hash map and looking up complement = target − current on each step. Real-time example: Checkout promo: given item prices [12, 7, 3, 9] and gift-card balance 16, return indices of 7 and 9. Pros: O(n) time, single pass, simple invariant to explain in interviews. Cons: O(n) extra memory; streaming with tiny RAM may force sort + two-pointer tradeoffs. Common mistakes: Reusing the same index; storing before checking complement (allows self-pair). Best practices: Clarify uniqueness assumption; mention index vs value return requirement upfront. Audience level: Engineering Manager.
Full explanation
Find two indices whose values sum to target in one pass by storing value→index in a hash map and looking up complement = target − current on each step.
Real example & use case
Checkout promo: given item prices [12, 7, 3, 9] and gift-card balance 16, return indices of 7 and 9.
Pros & cons
Pros: O(n) time, single pass, simple invariant to explain in interviews. Cons: O(n) extra memory; streaming with tiny RAM may force sort + two-pointer tradeoffs.
Common mistakes
Reusing the same index; storing before checking complement (allows self-pair).
Best practices
Clarify uniqueness assumption; mention index vs value return requirement upfront.
Follow-up questions
- How would you test Two Sum with Hash Map?
- What metrics prove Two Sum with Hash Map is healthy in prod?
- How does Two Sum with Hash Map change at 10× traffic?