Array Questions at American Express: What to Expect

Array questions dominate the technical interview at American Express, making up roughly two-thirds of their coding problems. This focus isn't arbitrary. Financial transactions, customer data streams, and ledger entries are fundamentally processed as sequences. Whether it's analyzing spending patterns, batch-processing transactions, or managing sorted lists of charges, arrays are the core data structure for these operations. Mastering array manipulation demonstrates you can handle the ordered, index-based data that underpins Amex's systems efficiently and correctly.

What to Expect — Types of Problems

You can expect array questions that test fundamental algorithmic thinking applied to practical scenarios. The problems typically fall into these categories:

• Two Pointers & Sliding Window: Essential for finding subarrays, comparing sequences, or handling sorted data. Expect problems related to transaction analysis, like finding a time window with maximum spending.
• Prefix Sum & Hashing: Used for problems involving subarray sums or finding pairs/triplets that meet a condition (e.g., finding transactions that sum to a specific amount).
• In-place Array Manipulation: Tasks like removing duplicates, moving zeros, or rotating arrays test your ability to modify data efficiently without extra space, a key skill for optimizing financial data processing.
• Sorting & Searching: While sometimes straightforward, these problems often serve as the first step for more complex challenges, such as merging intervals or finding missing elements in a sequence.

The problems are generally of medium difficulty, emphasizing clean, optimal solutions over obscure tricks.

How to Prepare — Study Tips with One Code Example

Focus on patterns, not memorization. Learn the core techniques (two pointers, sliding window, hashing) and practice applying them. For each problem, prioritize:

1. Stating the brute force approach and its complexity.
2. Identifying the optimal pattern.
3. Writing clean, runnable code.
4. Walking through test cases.

A key pattern is the Two-Pointer technique for in-place element removal. Here’s how to remove all instances of a val from an array in-place, returning the new length.

def removeElement(nums, val):
    k = 0  # Pointer for the next position of a non-val element
    for i in range(len(nums)):
        if nums[i] != val:
            nums[k] = nums[i]
            k += 1
    return k

Recommended Practice Order

Build your skills progressively:

1. Start with fundamental in-place operations (remove duplicates, move zeros).
2. Master the two-pointer technique for problems like two-sum (sorted) and container with most water.
3. Practice sliding window for subarray problems (maximum sum, fixed-size window).
4. Tackle prefix sum and hashing patterns for subarray sum problems.
5. Combine patterns for more complex challenges, like merging intervals or product of array except self.

This structured approach ensures you build the necessary muscle memory to recognize and implement the right solution under interview pressure.

Practice Array at American Express