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.
Showing 12 problems of 25
- #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 → - #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 → - #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 → - #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 → - #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 → - #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 → - #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 → - #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 → - #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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