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 22 problems of 100
- #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.
- #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.
- #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.
- #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.
- #30easysometimes asked
30. Invert Binary Tree
Invert (mirror) a binary tree. Plaid asks this as a recursion warm-up before harder category-tree transformations.
- #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.