Two Sum with Hash Map
As a Senior 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.
Audio N/AQuick interview answer
At Senior 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 Senior 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: Senior.
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?