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.
- #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.
- #5easysometimes asked
5. Remove Element
Remove all occurrences of a value from an array in-place and return the new length. Plaid asks this because filtering out pending or void transactions before ETL is exactly the same shape.
- #6easysometimes asked
6. Search Insert Position
Find the index where a target value should be inserted into a sorted array. Plaid asks this because finding the slot to splice a new ledger entry into a sorted journal is exactly this primitive.
- #7easysometimes asked
7. Plus One
Increment a non-negative integer represented as an array of digits. Plaid asks this because incrementing big-integer balances (cents stored as digit arrays) without overflow is a real ledger primitive.
- #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.
- #9easysometimes asked
9. Binary Tree Inorder Traversal
Return the inorder traversal of a binary tree's node values. Plaid asks this as a tree-recursion baseline before harder traversal problems on transaction-category hierarchies.
- #10easysometimes asked
10. Same Tree
Check if two binary trees are identical in structure and values. Plaid asks this as a baseline for harder tree-equality problems like checking whether two transaction-category snapshots match.
- #11easysometimes asked
11. Symmetric Tree
Check whether a binary tree is a mirror of itself. Plaid asks this to test mirrored-recursion fluency before harder graph-symmetry problems.
- #12easysometimes asked
12. Maximum Depth of Binary Tree
Return the maximum depth of a binary tree. Plaid asks this as a baseline for harder tree-depth problems on category hierarchies of varying depths from different financial institutions.
- #13easysometimes asked
13. Balanced Binary Tree
Determine if a binary tree is height-balanced. Plaid asks this because their internal category trees must stay balanced for predictable lookup latency — and detecting imbalance is the prerequisite for triggering a rebuild.
- #14easysometimes asked
14. Minimum Depth of Binary Tree
Return the minimum depth of a binary tree — the shortest root-to-leaf path. Plaid asks this because finding the nearest leaf in a category tree is exactly how they pick the shallowest classification for new transactions.
- #15easyrarely asked
15. Pascal's Triangle
Generate the first numRows of Pascal's triangle. Plaid asks this as a DP-warm-up that exercises the 'each row depends on the previous' pattern used in their rolling-window financial aggregations.
- #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.
- #17easysometimes asked
17. Valid Palindrome
Determine if a string is a palindrome after removing non-alphanumerics and lowercasing. Plaid asks this as a string-normalization warm-up before harder merchant-name canonicalization problems.
- #18easysometimes asked
18. Single Number
Find the number that appears exactly once in an array where every other number appears twice. Plaid asks this because finding the un-matched ledger entry in a reconciliation pass has exactly this shape.
- #19easysometimes asked
19. Linked List Cycle
Detect whether a linked list has a cycle. Plaid asks this as a baseline before harder graph-cycle problems on account-link chains and OAuth refresh-token graphs.
- #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.
- #21easysometimes asked
21. Two Sum II - Input Array Is Sorted
Given a sorted array, find two indices whose values sum to a target. Plaid asks this to verify you exploit sortedness — same shape as finding two transactions on a sorted balance-history that net to a target gap.
- #22easysometimes asked
22. Majority Element
Find the element that appears more than n/2 times in an array. Plaid asks this because identifying the dominant merchant category across a user's transactions has the same shape.
- #23easysometimes asked
23. Rotate Array
Rotate an array to the right by k steps in-place. Plaid asks this because shifting a circular buffer of recent transactions by a rotation amount has the same shape.
- #24easyrarely asked
24. Reverse Bits
Reverse the bits of a 32-bit unsigned integer. Plaid asks this as a bitwise-fluency check before harder hash-collision and idempotency-key problems.
- #25easyrarely asked
25. Number of 1 Bits
Count the number of set bits in an unsigned integer. Plaid asks this as a bitwise warm-up before harder bitmask problems on account-feature flags.
- #26easyrarely asked
26. Happy Number
Determine whether a number eventually reaches 1 under repeated digit-square-sum, or cycles forever. Plaid asks this because cycle detection on transformation sequences is the same algorithm as detecting OAuth refresh-token loops.
- #27easysometimes asked
27. Isomorphic Strings
Determine if two strings are isomorphic — one character maps to one other character consistently. Plaid asks this because tokenizing merchant strings under a consistent mapping is the same 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.
- #30easysometimes asked
30. Invert Binary Tree
Invert (mirror) a binary tree. Plaid asks this as a recursion warm-up before harder category-tree transformations.
- #31mediumfrequently asked
31. Add Two Numbers
Add two numbers represented as reverse-order linked lists. Plaid asks this because adding two arbitrary-precision balances digit-by-digit with carry is exactly how their ledger code handles cents that overflow JS Number precision.
- #32mediumfrequently asked
32. Longest Substring Without Repeating Characters
Find the length of the longest substring with no repeated characters. Plaid asks this as a sliding-window baseline before harder webhook-deduplication-window problems.
- #33mediumsometimes asked
33. Longest Palindromic Substring
Find the longest palindromic substring. Plaid asks this as a 'expand around center' fluency check before harder string-canonicalization problems on merchant names.
- #34mediumsometimes asked
34. Container With Most Water
Find two lines that, together with the x-axis, form the container with the most water. Plaid asks this as a two-pointer optimization classic before harder window problems on balance-history charts.
- #35mediumfrequently asked
35. 3Sum
Find all unique triplets that sum to zero. Plaid asks this because three-way reconciliation across a debit, a credit, and a fee is exactly this primitive — find three rows that net to zero.
- #36mediumrarely asked
36. Letter Combinations of a Phone Number
Generate all letter combinations that a phone-number string could represent. Plaid asks this as a backtracking warm-up before harder combinatorial problems on transaction-tag suggestions.
- #37mediumsometimes asked
37. Remove Nth Node From End of List
Remove the nth node from the end of a linked list in one pass. Plaid asks this as a two-pointer baseline before harder windowed-removal problems on transaction-history lists.
- #38mediumsometimes asked
38. Generate Parentheses
Generate all combinations of n well-formed parentheses. Plaid asks this as a backtracking pattern problem before harder nested-JSON validation work on webhook payloads.
- #39mediumsometimes asked
39. Swap Nodes in Pairs
Swap every two adjacent nodes in a linked list. Plaid asks this to verify pointer-juggling fluency before harder list-restructuring problems on chronological transaction sequences.
- #40mediumrarely asked
40. Next Permutation
Rearrange numbers into the lexicographically next greater permutation. Plaid asks this because deterministic enumeration of equivalent reorderings is the same primitive they use when retrying batch ETL jobs in a fixed cycle.
- #41mediumfrequently asked
41. Search in Rotated Sorted Array
Search for a target in a sorted array that has been rotated. Plaid asks this because querying a circular buffer of transaction timestamps that wraps around a clock pivot is the same primitive.
- #42mediumsometimes asked
42. Find First and Last Position of Element in Sorted Array
Find the first and last occurrence of a target in a sorted array. Plaid asks this because finding the time-range of all transactions for a given merchant in a sorted ledger is the same primitive.
- #43mediumsometimes asked
43. Valid Sudoku
Determine if a 9x9 Sudoku board is valid (without solving it). Plaid asks this as a constraint-validation primitive — the same shape as their multi-constraint webhook payload validator.
- #44mediumsometimes asked
44. Combination Sum
Find all unique combinations of candidates that sum to a target, where the same number may be chosen unlimited times. Plaid asks this because finding all ways to assemble a target balance from a fixed denomination set is the same primitive.
- #45mediumsometimes asked
45. Permutations
Return all possible permutations of distinct integers. Plaid asks this as a backtracking warm-up before harder ordering problems on transaction-processing schedules.
- #46mediumsometimes asked
46. Rotate Image
Rotate an NxN matrix 90 degrees clockwise in place. Plaid asks this because in-place 2D transforms are the same shape as their column-major to row-major balance-history transposition for charts.
- #47mediumfrequently asked
47. Group Anagrams
Group anagrams together from an array of strings. Plaid asks this because grouping merchant names that share the same character profile (after normalization) is a daily reality on their merchant-deduplication pipeline.
- #48mediumfrequently asked
48. Maximum Subarray
Find the contiguous subarray with the largest sum. Plaid asks this because finding the highest-net-deposit window in a transaction series uses exactly this primitive — Kadane's algorithm.
- #49mediumsometimes asked
49. Jump Game
Determine if you can reach the last index of an array where each value is your max jump length. Plaid asks this as a greedy-baseline problem before harder reachability questions on retry-graph traversal.
- #50mediumfrequently asked
50. Merge Intervals
Merge overlapping intervals. Plaid asks this because consolidating overlapping batch-windows (when ETL retries cover overlapping time ranges) is exactly this primitive.
- #51mediumsometimes asked
51. Unique Paths
Count the number of unique paths from top-left to bottom-right of an m x n grid, moving only right or down. Plaid asks this as a DP warm-up before harder grid-DP problems on transaction-state machines.
- #52mediumsometimes asked
52. Minimum Path Sum
Find the path from top-left to bottom-right with the smallest sum. Plaid asks this as a grid DP variation, mapping cleanly to lowest-cost routing in their multi-hop payment-rail graph.
- #53mediumsometimes asked
53. Simplify Path
Simplify a Unix-style absolute file path. Plaid asks this as a stack warm-up before harder hierarchical-category-tree normalization problems.
- #54mediumfrequently asked
54. Edit Distance
Find the minimum operations to convert one string into another (insert, delete, replace). Plaid asks this because fuzzy merchant matching uses edit distance to collapse variants like 'STARBUCKS #234' and 'Starbucks #234A' into one canonical merchant.
- #55mediumsometimes asked
55. Set Matrix Zeroes
If an element in an MxN matrix is 0, zero out its entire row and column in-place. Plaid asks this because invalidating a row+column band of a cached pivot table when one cell flips is exactly this primitive.
- #56mediumsometimes asked
56. Search a 2D Matrix
Search for a target in a row-sorted matrix where each row's first element is greater than the previous row's last. Plaid asks this as a binary-search-on-flattened-grid problem before harder bank-feed lookup questions.
- #57mediumsometimes asked
57. Sort Colors
Sort an array of 0s, 1s, and 2s in place in one pass. Plaid asks this because partitioning a transaction stream into 'pending', 'cleared', 'failed' buckets uses exactly this Dutch National Flag primitive.
- #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.
- #59mediumsometimes asked
59. Subsets
Return all possible subsets (the power set) of a distinct-integer array. Plaid asks this as a combinatorial-generation warm-up before harder subset-sum reconciliation problems.
- #60mediumsometimes asked
60. Word Search
Determine if a word exists in a 2D grid by traversing adjacent cells. Plaid asks this as a DFS-on-grid baseline before harder graph-traversal problems on account-link graphs.
- #61mediumrarely asked
61. Remove Duplicates from Sorted Array II
Remove duplicates in-place such that each element appears at most twice. Plaid asks this as a two-pointer variant — exactly the shape they use to enforce 'at most 2 retries per transaction in the same minute' logs.
- #62mediumrarely asked
62. Search in Rotated Sorted Array II
Search a rotated sorted array that may contain duplicates. Plaid asks this as a binary-search-with-ambiguity problem — exactly the shape they face when a circular log has duplicate-event timestamps.
- #63mediumrarely asked
63. Remove Duplicates from Sorted List II
Delete all nodes in a sorted linked list that have duplicate values, keeping only unique values. Plaid asks this because removing transactions that have any duplicate occurrences (not keeping one) is a stricter dedup needed for audit logs.
- #64mediumrarely asked
64. Partition List
Partition a linked list around a value x. Plaid asks this because partitioning a transaction stream into 'below threshold' and 'above threshold' is the same primitive.
- #65mediumsometimes asked
65. Decode Ways
Count the number of ways to decode a digit string where 'A'=1 ... 'Z'=26. Plaid asks this as a DP warm-up before harder parser-state problems on variable-length transaction-code formats.
- #66mediumsometimes asked
66. Reverse Linked List II
Reverse a sublist of a linked list between positions left and right. Plaid asks this as a pointer-juggling problem — exactly the shape they use when restoring a corrupted segment of a transaction-history list from a backup.
- #67mediumrarely asked
67. Restore IP Addresses
Generate all valid IP addresses from a digit string. Plaid asks this as a backtracking warm-up — the same shape they use when parsing variable-length transaction codes split across delimiters.
- #68mediumrarely asked
68. Unique Binary Search Trees II
Generate all structurally unique BSTs with n nodes. Plaid asks this as a recursion + memoization warm-up before harder tree-construction problems on category-tree variants.
- #69mediumfrequently asked
69. Validate Binary Search Tree
Determine if a binary tree is a valid BST. Plaid asks this because validating that a freshly-built merchant-category tree obeys its strict-ordering invariant is the same primitive — bugs in the invariant cause cascading mis-classification later.
- #70mediumfrequently asked
70. Binary Tree Level Order Traversal
Return the level-order traversal of a binary tree's values. Plaid asks this as a BFS baseline before harder graph-walks on payment-rail dependency trees.
- #71mediumsometimes asked
71. Binary Tree Zigzag Level Order Traversal
Return the zigzag level order traversal of a tree. Plaid asks this as a BFS variant — exactly the shape they use when alternating left-to-right vs right-to-left iteration over partitioned ledger shards.
- #72mediumsometimes asked
72. Construct Binary Tree from Preorder and Inorder Traversal
Reconstruct a binary tree from its preorder and inorder traversals. Plaid asks this because deserializing a tree from two stream snapshots (preorder = parent-first, inorder = sorted) is the same reconstruction primitive.
- #73easysometimes asked
73. Convert Sorted Array to Binary Search Tree
Convert a sorted array into a height-balanced BST. Plaid asks this because indexing a sorted financial-category list into a balanced lookup tree for O(log n) range queries is exactly the same primitive.
- #74mediumfrequently asked
74. LRU Cache
Design an LRU cache with O(1) get and put. Plaid asks this because their idempotency-key store and recent-transactions cache both need O(1) bounded-memory LRU semantics.
- #75mediumfrequently asked
75. Word Break
Determine if a string can be segmented into a sequence of dictionary words. Plaid asks this because tokenizing a merchant string into known sub-tokens (e.g., 'AMZNMKTPLACE' -> 'AMZN' + 'MKT' + 'PLACE') is the same primitive.
- #76mediumsometimes asked
76. Copy List with Random Pointer
Deep copy a linked list where each node has a random pointer to any other node. Plaid asks this because cloning an entity graph with cross-references (transactions pointing to merchants pointing to categories) uses the same pattern.
- #77mediumfrequently asked
77. Clone Graph
Deep copy a connected undirected graph. Plaid asks this because their account-link graph (user -> bank -> account -> transaction) is exactly this kind of cyclic reference structure that must be cloneable for snapshot tests.
- #78mediumsometimes asked
78. Sort List
Sort a linked list in O(n log n) time. Plaid asks this because sorting a stream of transactions that arrived as a linked list (from a bank-feed batch) before merging into the master ledger is exactly this primitive.
- #79mediumsometimes asked
79. Find Minimum in Rotated Sorted Array
Find the minimum element in a rotated sorted array. Plaid asks this because finding the rotation pivot (e.g., the day a circular billing window restarts) is the same primitive.
- #80mediumrarely asked
80. Find Peak Element
Find a peak element in an array (one whose value is strictly greater than its neighbors). Plaid asks this as a binary-search-on-properties problem — exactly the shape they use to find traffic peaks in a time-bucketed transaction-rate series.
- #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.
- #92mediumsometimes asked
92. Insert Interval
Insert a new interval into a sorted, non-overlapping list and merge if necessary. Plaid asks this because adding a new ETL-retry window to a sorted list of pre-existing windows is exactly this primitive.
- #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.
- #98mediumsometimes asked
98. Recover Binary Search Tree
Two nodes in a BST have been swapped by mistake; recover the tree without changing structure. Plaid asks this because detecting and repairing two-row swap corruption in a sorted category tree (without rebuilding) is exactly this primitive.
- #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.