Prefix Sum Questions at Expedia: What to Expect

Prefix Sum is a non-negotiable skill for Expedia interviews. With 4 out of their 54 tracked coding questions explicitly testing this pattern, it appears in roughly 7-8% of their problem set. For a company that processes vast amounts of transactional and location-based data (think pricing arrays, daily booking totals, or geographic queries), the ability to compute range sums and aggregates in constant time is directly applicable. Mastering it demonstrates you can optimize core operations on sequential data, a valuable trait for building scalable travel platforms.

What to Expect — Types of Problems

Expedia’s Prefix Sum questions typically fall into two categories. First, direct range-sum queries on a one-dimensional array. You’ll be given a list of numbers and asked to answer multiple queries for the sum between indices i and j. The brute-force approach fails at scale, making the Prefix Sum optimization the clear solution.

Second, and more common at Expedia, are problems that transform into a Prefix Sum problem. These involve tracking cumulative counts or balances to find subarrays meeting a specific condition, such as finding the longest subarray with a sum equal to k, or problems dealing with averages or balances. The key is recognizing when a problem’s core challenge—like finding a zero-sum subarray or calculating a running metric—can be reframed as querying a precomputed prefix array.

How to Prepare — Study Tips with One Code Example

Start by internalizing the core formula: prefix[i] represents the sum of elements from arr[0] to arr[i-1]. The sum of any subarray arr[left...right] is prefix[right+1] - prefix[left]. Practice deriving this until it’s automatic.

The most critical step is pattern identification. Look for prompts with keywords like "subarray sum," "continuous subarray," "range query," or "cumulative." When you see a need for repeated sum calculations over ranges, Prefix Sum is your first suspect.

Here is the essential pattern for building a prefix sum array and answering a range query:

def prefix_sum_queries(arr, queries):
    # Build prefix sum array. prefix[0] = 0 for easier calculation.
    n = len(arr)
    prefix = [0] * (n + 1)
    for i in range(n):
        prefix[i + 1] = prefix[i] + arr[i]

    # Answer each query [l, r] (0-indexed inclusive)
    results = []
    for l, r in queries:
        # sum(arr[l..r]) = prefix[r+1] - prefix[l]
        results.append(prefix[r + 1] - prefix[l])
    return results

Recommended Practice Order

1. Master the Fundamentals: Solve classic range sum query problems (LeetCode 303, 304).
2. Tackle Subarray Problems: Practice identifying Prefix Sum in disguise (LeetCode 560, 525, 325). These are highly relevant to Expedia.
3. Apply to Variations: Solve problems involving averages, balances, or with a slight twist (LeetCode 209, 238).
4. Simulate Company-Specific Pressure: Finally, time yourself on Expedia-tagged Prefix Sum questions in a mock interview setting.

Practice Prefix Sum at Expedia