Array Questions at IBM: What to Expect

Array questions make up over half of IBM's technical interview problem set (94 out of 170). This isn't an accident. Arrays are the fundamental data structure for representing ordered data, and IBM's work—from database systems and mainframe software to cloud analytics and AI—often involves processing large, sequential datasets. Successfully manipulating arrays demonstrates core skills in logic, efficiency, and systematic thinking, which are critical for the systems-level and data-intensive programming common at IBM.

What to Expect — Types of Problems

IBM's array questions tend to focus on practical application rather than obscure tricks. You can generally expect problems in these categories:

1. In-Place Manipulation: Tasks that require modifying the array without using extra space, such as moving zeros, removing duplicates from a sorted array, or applying rotations. These test your understanding of pointer mechanics and careful element swapping.
2. Subarray Analysis: Problems involving contiguous segments, like finding the maximum sum subarray (Kadane's Algorithm), subarrays with a given sum, or fixed-size sliding window problems. These are common in data stream processing scenarios.
3. Sorting & Searching Variations: Implementing or leveraging sorting (merge sort, quicksort concepts) and modified binary search on rotated or nearly-sorted arrays. These assess your grasp of fundamental algorithms.
4. Two-Pointer & Multi-Pointer Techniques: Solving problems like two-sum, three-sum, or merging sorted arrays by using indices to traverse the array from different points. This pattern is ubiquitous.
5. Basic Matrix Operations: Since a matrix is a 2D array, expect some questions on traversing or searching within a grid (e.g., spiral order, search in a row-wise and column-wise sorted matrix).

How to Prepare — Study Tips with One Code Example

Focus on mastering patterns, not memorizing solutions. For each problem type above, learn the core algorithmic approach. Then, practice implementing it from scratch until you can do it without hesitation. Always analyze the time and space complexity of your solution.

A key pattern to internalize is the Two-Pointer Technique. It's used for in-place operations, searching, and comparing elements. Here is a classic example: removing duplicates from a sorted array in-place, returning the new length.

def removeDuplicates(nums):
    if not nums:
        return 0
    write_index = 1
    for read_index in range(1, len(nums)):
        if nums[read_index] != nums[read_index - 1]:
            nums[write_index] = nums[read_index]
            write_index += 1
    return write_index

The pattern uses a read_index to scan the array and a write_index to place the next unique element. This runs in O(n) time and O(1) space.

Recommended Practice Order

Build your skills progressively:

1. Start with fundamental in-place operations (e.g., remove element, move zeros).
2. Move to two-pointer problems (two-sum, reverse string/array).
3. Tackle sliding window and subarray problems.
4. Practice sorting and searching variations.
5. Finally, combine patterns in more complex problems (e.g., three-sum uses sorting + two-pointer).

This order ensures you build the component skills needed for harder challenges.

Practice Array at IBM