IBM Coding Interview Questions
25 IBM coding interview problems with full optimal solutions — 12 easy, 13 medium, 0 hard. Every problem ships with multiple approaches (brute-force first, then the optimal), complexity tables for each, company-specific tips on what an IBM interviewer values, and a FAQ section.
- #1easyfoundational
1. Two Sum
Two Sum is IBM's universal warm-up across SWE and intern phone screens. The interviewer is grading whether you can pivot from the obvious O(n^2) double loop to the one-pass hash-map solution and whether you can articulate the space-for-time tradeoff cleanly.
3 free resourcesSolve → - #3mediumfrequently asked
3. Longest Substring Without Repeating Characters
Longest Substring Without Repeating Characters is IBM's sliding-window screener. The interviewer is grading whether you reach for the dynamic-window technique with a last-seen index map, whether you handle the 'jump the left pointer' optimization, and whether you ship O(n) with a single pass.
3 free resourcesSolve → - #15mediumfrequently asked
15. 3Sum
3Sum is IBM's two-pointer-on-sorted-array screener. The interviewer is grading whether you sort first to enable the two-pointer pattern, whether you skip duplicates cleanly at all three pointer positions, and whether you arrive at O(n^2) instead of the naive O(n^3).
3 free resourcesSolve → - #20easyfoundational
20. Valid Parentheses
Valid Parentheses is IBM's stack-pattern screener. The interviewer is grading whether you recognize the stack signature (matching pairs of arbitrary depth), whether you handle the closer-without-opener and unclosed-tail edge cases, and whether you finish in O(n) on a single pass.
3 free resourcesSolve → - #21easyfoundational
21. Merge Two Sorted Lists
Merge Two Sorted Lists is IBM's pointer-discipline check for SWE-1 and intern phone screens. The interviewer is grading whether you reach for a sentinel/dummy node, whether you re-use existing nodes instead of allocating fresh ones, and whether your loop exit cleanly attaches the remaining tail.
3 free resourcesSolve → - #49mediumfrequently asked
49. Group Anagrams
Group Anagrams is IBM's hash-keyed-bucketing screener. The interviewer is grading whether you pick a canonical key (sort the chars OR a frequency tuple), articulate the tradeoff between O(k log k) sort and O(k) counter, and ship the bucket map cleanly.
3 free resourcesSolve → - #53mediumfrequently asked
53. Maximum Subarray
Maximum Subarray is IBM's Kadane's-algorithm screener. The interviewer is grading whether you can derive Kadane's by stating the 'extend-or-restart' decision rule, ship it in O(n) with O(1) space, and articulate how it generalizes to the 2D variant.
4 free resourcesSolve → - #56mediumfrequently asked
56. Merge Intervals
Merge Intervals is IBM's sort-then-sweep screener — directly relevant to calendar scheduling and time-range queries in IBM Cloud monitoring. The interviewer is grading whether you sort by start, whether you handle the touching-but-not-overlapping edge case, and whether you ship O(n log n) cleanly.
3 free resourcesSolve → - #70easyfrequently asked
70. Climbing Stairs
Climbing Stairs is IBM's intro-to-DP screener — the candidate's first chance to demonstrate they recognize Fibonacci, can derive the recurrence on the whiteboard, and can collapse the memoization to two scalar variables for O(1) space.
3 free resourcesSolve → - #102mediumfrequently asked
102. Binary Tree Level Order Traversal
Binary Tree Level Order Traversal is IBM's BFS-on-trees screener. The interviewer is grading whether you reach for an explicit queue, whether you use the queue-size snapshot trick to delimit levels, and whether you handle the empty-tree edge case cleanly.
3 free resourcesSolve → - #121easyfoundational
121. Best Time to Buy and Sell Stock
Best Time to Buy and Sell Stock is IBM's array-traversal screener for SWE phone screens and the Watson/Research data-engineering track. The interviewer is grading whether you can derive the running-minimum invariant on a single pass instead of falling into the brute-force double loop.
3 free resourcesSolve → - #125easyfoundational
125. Valid Palindrome
Valid Palindrome is IBM's go-to string-parsing screener. The bar is whether you can articulate the two-pointer in-place approach, handle non-alphanumeric characters in the spec, and avoid the O(n) extra-space trap of building a normalized copy.
3 free resourcesSolve → - #136easyfrequently asked
136. Single Number
Single Number is IBM's XOR-trick screener. The interviewer is grading whether you reach for the bitwise XOR pattern, articulate why it works (a XOR a = 0, a XOR 0 = a, XOR is commutative), and ship it in O(n) time with O(1) space.
3 free resourcesSolve → - #139mediumfrequently asked
139. Word Break
Word Break is IBM's DP-on-strings screener. The interviewer is grading whether you recognize the 'can-be-split' DP recurrence, whether you build the Set for O(1) word lookup, and whether you bound the inner loop by the maximum word length to avoid pathological inputs.
3 free resourcesSolve → - #141easyfrequently asked
141. Linked List Cycle
Linked List Cycle is IBM's Floyd's-tortoise-and-hare screener. The interviewer is grading whether you reach for the two-pointer technique unprompted, articulate why the fast pointer catches the slow one inside a cycle, and ship O(1) extra space.
4 free resourcesSolve → - #146mediumcompany favorite
146. LRU Cache
LRU Cache is IBM's data-structure-design screener — relevant to IBM Cloud / DB2 / storage teams where eviction policies matter daily. The interviewer is grading whether you reach for hash map + doubly linked list together, articulate why neither suffices alone, and ship O(1) for both get and put.
4 free resourcesSolve → - #200mediumfrequently asked
200. Number of Islands
Number of Islands is IBM's graph-traversal screener — a 2D grid where each connected region of land cells counts as one island. The interviewer is grading whether you pick BFS or DFS deliberately, articulate the stack-depth risk on huge maps, and handle the in-place vs visited-set trade.
3 free resourcesSolve → - #206easyfoundational
206. Reverse Linked List
Reverse Linked List is IBM's canonical pointer warm-up across SWE phone screens. The interviewer is grading whether you can walk a singly linked list with three pointers without losing the tail and whether you can articulate the recursive variant's stack cost.
3 free resourcesSolve → - #207mediumfrequently asked
207. Course Schedule
Course Schedule is IBM's cycle-detection-in-directed-graph screener — directly relevant to dependency-graph problems in IBM Cloud orchestration and DB2 query planning. The interviewer is grading whether you reach for topological sort (Kahn's or DFS-coloring), whether you handle disconnected nodes, and whether you finish in O(V+E).
4 free resourcesSolve → - #215mediumfrequently asked
215. Kth Largest Element in an Array
Kth Largest Element is IBM's heap/quickselect screener. The interviewer is grading whether you ship the O(n log k) min-heap solution under time pressure, whether you mention Quickselect for O(n) average, and whether you handle the duplicate-friendly definition correctly.
4 free resourcesSolve → - #217easyfoundational
217. Contains Duplicate
Contains Duplicate is IBM's hash-set screener for SWE interns and SWE-1 phone screens. The interviewer is grading whether you reach for a Set in one pass, whether you can short-circuit early, and whether you name the sort vs hash space-time tradeoff cleanly.
3 free resourcesSolve → - #238mediumfrequently asked
238. Product of Array Except Self
Product of Array Except Self is IBM's prefix/suffix-array screener. The interviewer is grading whether you avoid the forbidden division operator, derive the two-pass left/right product technique, and ship it in O(1) extra space using the output array as your scratch.
3 free resourcesSolve → - #242easyfrequently asked
242. Valid Anagram
Valid Anagram is IBM's hash-map screener. The interviewer is grading whether you can pick the right counting structure (array vs Map), handle the Unicode follow-up gracefully, and stay O(n) instead of falling into the O(n log n) sort trap.
3 free resourcesSolve → - #322mediumfrequently asked
322. Coin Change
Coin Change is IBM's unbounded-knapsack DP screener. The interviewer is grading whether you recognize the unbounded variant (each coin reusable), whether you ship the O(amount * coins) bottom-up DP, and whether you handle the impossible case (amount > 0 but no combination works) cleanly.
3 free resourcesSolve → - #704easyfoundational
704. Binary Search
Binary Search is IBM's correctness screener — the interviewer is grading whether you can write a bug-free binary search on the whiteboard, handle the overflow-safe midpoint, and articulate the loop-invariant. Surprisingly few candidates ship this on the first attempt.
4 free resourcesSolve →
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