Plaid Coding Interview Questions
100 Plaid coding interview problems with full optimal solutions — 31 easy, 50 medium, 19 hard. Every problem ships with multiple approaches (brute-force first, then the optimal), complexity tables for each, company-specific tips on what an Plaid interviewer values, and a FAQ section.
Showing 9 problems of 100
- #1easyfrequently asked
1. Two Sum
Given an array of integers and a target, return indices of two numbers that sum to the target. Plaid asks this as a warm-up before harder ETL problems to verify you reach for a hash map rather than the nested-loop instinct.
- #2easyfrequently asked
2. Valid Parentheses
Determine if a string of brackets is balanced and properly nested. Plaid uses this to test stack reflexes before moving to nested JSON validation in webhook payloads.
- #3easyfrequently asked
3. Merge Two Sorted Lists
Merge two sorted linked lists into one sorted list. Plaid asks this because merging two pre-sorted streams of transactions from different bank feeds is a daily operation on their ETL pipeline.
- #4easyfrequently asked
4. Remove Duplicates from Sorted Array
In-place de-duplicate a sorted array and return the new length. Plaid asks this because de-duping near-duplicate transactions (same id, same amount, slight timestamp drift) is a daily reality on their ingestion pipeline.
- #8easyfrequently asked
8. Merge Sorted Array
Merge two sorted arrays in-place, where the first has extra trailing space to hold the second. Plaid asks this because merging a new batch of transactions into a pre-allocated buffer is a hot path in their ingestion code.
- #16easyfrequently asked
16. Best Time to Buy and Sell Stock
Find the maximum profit from buying and selling a stock once. Plaid asks this because computing max-drawdown style metrics over a price series is a familiar shape for their balance-history endpoints.
- #20easyfrequently asked
20. Min Stack
Design a stack that supports push, pop, top, and retrieving the minimum element in constant time. Plaid asks this because tracking the running minimum balance over a transaction stream needs exactly this primitive.
- #28easyfrequently asked
28. Reverse Linked List
Reverse a singly-linked list. Plaid asks this as a pointer-juggling baseline before harder list problems on transaction history sequences.
- #29easyfrequently asked
29. Contains Duplicate
Determine if any value appears at least twice in an array. Plaid asks this because detecting duplicate webhook deliveries or duplicate idempotency keys is exactly this primitive.