Array Questions at Grammarly: What to Expect

Array questions make up nearly half of Grammarly’s technical interview problem set—11 out of 26 total questions. This heavy emphasis isn’t arbitrary. Grammarly’s core product, a real-time writing assistant, processes vast streams of text data. Under the hood, this involves analyzing sequences: characters in words, tokens in sentences, and operations in editing buffers. Arrays (and their close relatives, strings) are the fundamental data structures for modeling these sequences. Mastering array manipulation is essential for any engineer working on Grammarly’s text analysis, suggestion algorithms, or performance-critical editing features. If you can’t efficiently traverse, transform, and analyze ordered data, you’ll struggle with the domain’s core challenges.

What to Expect — Types of Problems

Grammarly’s array questions tend to focus on practical application over abstract puzzle-solving. You can expect problems that test your ability to implement clean, efficient solutions to common sequence-manipulation tasks. Key categories include:

• In-Place Operations: Modifying an array without allocating significant extra space. This is crucial for memory efficiency in high-performance contexts.
• Two-Pointer Techniques: Using multiple indices to traverse an array in a single pass, often for tasks like partitioning, searching for pairs, or removing duplicates.
• Sliding Window: Tracking a subset (window) of elements as it moves across the array, ideal for problems involving contiguous subarrays, such as finding a maximum sum or a substring with certain properties.
• String Manipulation: Since strings are character arrays, many problems will present as text processing challenges, testing your comfort with array methods and indexing on string data.

The problems are designed to assess not just if you get the correct answer, but how you reason about edge cases, time/space complexity, and code clarity.

How to Prepare — Study Tips with One Code Example

Focus on mastering a few core patterns rather than memorizing hundreds of problems. Understand the when and why behind techniques like two-pointers or sliding windows. Practice implementing them from scratch until they become muscle memory. Always analyze the time and space complexity (Big O) of your solution and discuss trade-offs.

A fundamental pattern you must know is the Two-Pointer technique for in-place element removal. A common problem is: "Given a sorted array, remove duplicates in-place such that each unique element appears only once. Return the new length."

The efficient approach uses a slow pointer (i) to track the position of the last unique element and a fast pointer (j) to scan ahead.

def removeDuplicates(nums):
    if not nums:
        return 0
    i = 0  # slow pointer for unique elements
    for j in range(1, len(nums)):  # fast pointer
        if nums[j] != nums[i]:
            i += 1
            nums[i] = nums[j]
    return i + 1  # length of unique segment

Recommended Practice Order

1. Start with foundational in-place operations (e.g., removing elements, moving zeros).
2. Master the two-pointer technique in its various forms (converging pointers, slow/fast pointers).
3. Practice sliding window problems to get comfortable with dynamic subarray calculations.
4. Apply these patterns to string-specific problems, as they are highly relevant to Grammarly's domain.
5. Finally, tackle a few of Grammarly’s actual array problems to synthesize these skills in a context similar to the interview.

Practice Array at Grammarly