Pivot order-status counts
Company: Instacart
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: Medium
Interview Round: Onsite
##### Question
Given an orders table (order_id, shopper_id, order_date, status, …), pivot the data so that each row is one shopper and each column shows the count of orders for a specific status (e.g., DELIVERED, CANCELED, RETURNED). Provide an efficient Java solution that works on a platform without external date-parsing libraries.
Quick Answer: This interview question evaluates algorithm design, data structures, correctness, complexity, edge cases, and implementation details in a realistic interview setting. A strong answer for Pivot order-status counts states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.
Solution
# Solution Alignment
The prompt asks for an implementation-level answer. The safest way to present it is to define the state, maintain clear invariants, then walk through complexity and tests.
## Problem Restatement
##### Question Given an orders table (order_id, shopper_id, order_date, status, …), pivot the data so that each row is one shopper and each column shows the count of orders for a specific status (e.g., DELIVERED, CANCELED, RETURNED). Provide an efficient Java solution that works on a platform without external date-parsing libraries.
## Recommended Approach
Start with a brute-force baseline to confirm correctness, then identify the repeated work or ordering property that enables a better data structure such as a hash map, heap, stack, queue, two pointers, prefix sums, BFS/DFS, or dynamic programming. Write the implementation around a small invariant and test that invariant directly.
## Correctness
The implementation should maintain an invariant after each loop or operation that directly matches the problem statement. At termination, that invariant implies the returned value has considered every valid candidate exactly once, or has preserved the required data-structure state after every API call.
## Complexity
State the baseline complexity and the optimized complexity. For most interview constraints, justify why the optimized approach meets the expected input size.
## Edge Cases and Tests
Empty and singleton inputs, duplicates, ties, invalid inputs, boundary values, and tests that exercise the main invariant.