Two Pointers Questions at American Express: What to Expect

Two Pointers is a high-frequency pattern at American Express, appearing in 5 of their 24 tagged problems. This 20% representation signals it's a core technique for evaluating a candidate's ability to write efficient, clean code for array and string manipulation—skills directly applicable to processing financial transactions, validating data streams, or optimizing in-memory operations.

Mastering this pattern demonstrates you can move beyond brute-force solutions to implement O(n) algorithms with minimal extra space, a practical efficiency requirement in backend systems.

What to Expect — Types of Problems

American Express uses Two Pointers to test logical reasoning and edge-case handling. Expect problems in these categories:

1. Opposite-Ends Pointers: Used for sorted arrays or palindrome checks. One pointer starts at the beginning, the other at the end, moving inward based on a condition (e.g., finding a pair sum).
2. Fast & Slow Pointers: Used to detect cycles or find midpoints in linked lists, a common data structure in system design.
3. Sliding Window Variants: While often its own category, some sliding window problems are solved with a two-pointer approach that maintains a subarray. You may see these for substring problems.

The problems will often involve arrays, strings, or linked lists. The key is recognizing when the input is sorted or when you need to track two positions to avoid a nested loop.

How to Prepare — Study Tips with One Code Example

Focus on understanding the pattern, not memorizing solutions. Start by identifying the problem type: if it asks for pairs, palindromes, or subarrays/substrings with a specific condition, Two Pointers is a prime candidate.

Practice this process:

1. Clarify if the input is sorted. If not, can sorting it first unlock the pattern?
2. Decide on your pointer movement logic before coding.
3. Handle duplicates and empty input edge cases.

Here is a classic example: Checking if a string is a palindrome (ignoring non-alphanumeric characters and case). This uses opposite-end pointers.

def is_palindrome(s: str) -> bool:
    left, right = 0, len(s) - 1
    while left < right:
        # Move left pointer if not alphanumeric
        while left < right and not s[left].isalnum():
            left += 1
        # Move right pointer if not alphanumeric
        while left < right and not s[right].isalnum():
            right -= 1
        # Compare characters (case-insensitive)
        if s[left].lower() != s[right].lower():
            return False
        left += 1
        right -= 1
    return True

Recommended Practice Order

Build competency progressively:

1. Fundamentals: Start with basic opposite-ends pointers (Two Sum II - Input Array Is Sorted, Valid Palindrome).
2. Fast & Slow: Practice cycle detection in linked lists (Linked List Cycle).
3. In-place manipulation: Try problems where you must modify the input array (Remove Duplicates from Sorted Array).
4. Sliding window overlap: Practice problems where two pointers track a window (Minimum Size Subarray Sum).
5. Amex-specific: Finally, tackle the actual Two Pointers problems tagged for American Express to familiarize yourself with their problem style and difficulty.

Practice Two Pointers at American Express