Two Sum with Hash Map
Go deep on Two Sum with Hash Map: edge cases, scalability limits, and how you'd evolve the solution over 2 years.
Answers use simple, clear English.
Quick interview answer
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. Scale limits often appear in: O(n) extra memory; streaming with tiny RAM may force sort + two-pointer tradeoffs..
Detailed answer
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. Scale limits often appear in: O(n) extra memory; streaming with tiny RAM may force sort + two-pointer tradeoffs.. Evolution levers: Clarify uniqueness assumption; mention index vs value return requirement upfront.. Anchor example: Checkout promo: given item prices [12, 7, 3, 9] and gift-card balance 16, return indices of 7 and 9. 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: Tech Lead.
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
Open one as its own read / solve / listen card