Two Pointers Questions at eBay: What to Expect

Two Pointers questions appear in about 15% of eBay's technical interview problems (9 out of 60). This frequency is significant, indicating the technique is a reliable filter for assessing a candidate's ability to optimize solutions and manipulate data in sequences—skills directly relevant to eBay's work with search results, catalog data, and real-time user interactions.

What to Expect — Types of Problems

At eBay, Two Pointers problems typically fall into two categories, both focusing on efficiency over brute force.

1. Opposite-End Pointers: Used primarily on sorted arrays or strings. The pointers start at the beginning and end, moving toward each other. This is the classic pattern for problems like finding a pair with a target sum, checking for a palindrome, or reversing a sequence in-place. Expect to see variations that test your ability to handle duplicates or work with slightly modified conditions.

2. Fast & Slow (or Sliding Window) Pointers: Used for linked list cycles or subarray/substring problems. In a linked list, one pointer moves twice as fast to detect cycles. More commonly at eBay, this pattern morphs into a sliding window to find a contiguous subarray meeting a condition (e.g., maximum sum, shortest/longest substring with certain characters). This tests your skill in tracking window state and adjusting boundaries efficiently.

The common thread is the requirement to achieve O(n) time complexity with O(1) extra space, moving beyond naive O(n²) or O(n log n) approaches.

How to Prepare — Study Tips with One Code Example

Master the fundamentals before tackling eBay-specific problems. Start by perfectly coding the basic patterns from scratch.

• Internalize the Patterns: For opposite-end pointers, practice until initializing left=0, right=len(arr)-1 and the while left < right loop is automatic. For sliding window, drill the while or for loop structure that expands the right pointer and conditionally shrinks the left.
• Trace Manually: Use a whiteboard or paper. Step through the algorithm with sample input, updating pointer values and variables at each step. This builds intuition for edge cases.
• Focus on Clean Logic: Write code that is straightforward. Complex pointer logic is a common source of bugs. Prioritize readability over clever one-line tricks.

A core pattern to master is the opposite-end two-pointer sum check. Here is the clean implementation:

def has_target_sum(nums, target):
    left, right = 0, len(nums) - 1
    while left < right:
        current_sum = nums[left] + nums[right]
        if current_sum == target:
            return True
        elif current_sum < target:
            left += 1  # Need a larger sum
        else:
            right -= 1  # Need a smaller sum
    return False

Recommended Practice Order

Build competence progressively. Do not jump directly to hard problems.

1. Foundation: "Two Sum II" (sorted version) and "Valid Palindrome." These cement the basic opposite-end movement.
2. Sliding Window Introduction: "Maximum Average Subarray I" or "Minimum Size Subarray Sum." Learn to expand/contract a window.
3. Linked List: "Linked List Cycle" to practice the fast/slow runner pattern.
4. eBay-Level Complexity: Practice problems involving removal of duplicates in-place, container with most water, or substrings with distinct characters. These combine pointer movement with more complex state tracking.
5. Final Review: Simulate an interview by solving 2-3 medium-difficulty Two Pointers problems with a 30-minute time limit. Explain your reasoning aloud.

Practice Two Pointers at eBay