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 19 problems of 100
- #58hardfrequently asked
58. Minimum Window Substring
Find the smallest window in a string containing all characters of a pattern. Plaid asks this because finding the smallest time window covering all required transaction-types in a feed (e.g., debit, credit, fee) is the same primitive.
- #81hardsometimes asked
81. Median of Two Sorted Arrays
Find the median of two sorted arrays in O(log(min(m,n))). Plaid asks this as a binary-search-on-partitions problem because computing percentile across two sharded ledgers without merging them is the same primitive.
- #82hardsometimes asked
82. Regular Expression Matching
Implement regular expression matching with '.' and '*'. Plaid asks this because matching merchant-name patterns with wildcards is a recurring sub-problem in their pattern-based classification rules.
- #83hardfrequently asked
83. Merge k Sorted Lists
Merge k sorted linked lists into one sorted list. Plaid asks this because merging k pre-sorted bank-feed streams into a unified ledger is exactly this primitive — it's their core ETL fan-in operation.
- #84hardsometimes asked
84. Reverse Nodes in k-Group
Reverse the nodes of a linked list k at a time. Plaid asks this as an advanced pointer-juggling problem — the same shape they use when reordering chunks of a transaction-batch list during retry-coalescing.
- #85hardrarely asked
85. Substring with Concatenation of All Words
Find all starting indices of substrings that are concatenations of every word in a given list (each used exactly once, any order). Plaid asks this as a multi-token sliding-window problem — exactly the shape they use to find required-feature sequences in a webhook batch.
- #86hardrarely asked
86. Longest Valid Parentheses
Find the length of the longest valid parentheses substring. Plaid asks this as a stack-based scanning problem — the same shape they use to find the longest run of properly-paired JSON braces in a corrupted webhook payload.
- #87hardsometimes asked
87. First Missing Positive
Find the smallest missing positive integer in O(n) time and O(1) space. Plaid asks this because finding the next available transaction-id in a partially-allocated id-pool is exactly this primitive.
- #88hardfrequently asked
88. Trapping Rain Water
Compute how much water can be trapped between vertical bars. Plaid asks this as a two-pointer + running-max problem — the same shape they use to compute the cumulative un-allocated liquidity across a fluctuating balance series.
- #89hardrarely asked
89. Wildcard Matching
Implement wildcard matching with '?' (single char) and '*' (any sequence). Plaid asks this because matching merchant patterns with arbitrary-length wildcards is part of their pattern-based category rules.
- #90hardsometimes asked
90. Jump Game II
Find the minimum number of jumps to reach the last index. Plaid asks this as a BFS-as-greedy problem — the same shape as finding the minimum number of API-hops to reach a target endpoint across their bank-rail graph.
- #91hardrarely asked
91. N-Queens
Place N queens on an NxN board such that no two attack each other; return all solutions. Plaid asks this as a backtracking classic before harder constraint-satisfaction problems.
- #93hardrarely asked
93. Text Justification
Format text into fully justified lines of fixed width. Plaid asks this as a careful greedy + arithmetic problem — the same flavor of edge-case discipline they need when formatting tabular statement output for end users.
- #94hardrarely asked
94. Sudoku Solver
Solve a 9x9 Sudoku in place. Plaid asks this as a backtracking + constraint-propagation problem — the same shape as their constraint-satisfaction engine that validates merchant-category assignments under multi-axis rules.
- #95hardsometimes asked
95. Largest Rectangle in Histogram
Find the area of the largest rectangle in a histogram. Plaid asks this as a monotonic-stack problem — the same shape they use to find the longest sustained-throughput window in a transaction-rate histogram.
- #96hardrarely asked
96. Maximal Rectangle
Find the largest all-1's rectangle in a binary matrix. Plaid asks this as a 'reduce-to-known-problem' classic — the maximal rectangle in a binary matrix reduces to multiple histogram problems.
- #97hardsometimes asked
97. Valid Number
Determine if a string represents a valid number (with signs, decimals, exponents). Plaid asks this as a finite-state-machine problem — exactly the shape they need when parsing transaction amounts from heterogeneous bank-feed formats.
- #99hardsometimes asked
99. Word Ladder
Find the shortest transformation from beginWord to endWord, changing one letter at a time, where each intermediate is in the dictionary. Plaid asks this as a BFS-shortest-path problem — the same shape as finding the minimum-hop route across their bank-rail graph.
- #100hardfrequently asked
100. Serialize and Deserialize Binary Tree
Design serialize/deserialize for a binary tree. Plaid asks this because their merchant-category tree must be snapshot-able for caching and idempotent replay — they need a stable, deterministic serialization that survives round trips.