Stack Questions at Expedia: What to Expect

Stack questions appear in about 9% of Expedia's technical interview problems. While this may seem modest, these problems are often foundational to solving more complex scenarios involving nested structures, parsing, or state management—common in travel booking systems, itinerary validation, and pricing rule engines. Mastering stacks demonstrates your ability to handle sequential data with LIFO (Last-In, First-Out) logic, a key skill for backend and full-stack roles at the company.

What to Expect — Types of Problems

Expedia's stack problems typically fall into three categories:

1. Classic Algorithmic Problems: These test fundamental understanding. Expect variations of valid parentheses, stack-based queue or min-stack implementations, and next greater element problems.
2. Simulation & Parsing: These mirror real-world tasks. You might process a sequence of operations (like a simplified booking log), evaluate a reverse polish notation expression (similar to a rule engine), or simplify a file path string.
3. Hybrid Problems: These combine a stack with another technique. A common pattern is using a stack to maintain indices or states while traversing an array or string, often paired with greedy logic or used to find nested patterns.

The problems are designed to be solved within a 30-45 minute interview slot, so while they can be tricky, they are rarely overly convoluted.

How to Prepare — Study Tips with One Code Example

Focus on pattern recognition, not memorization. The core stack pattern involves iterating through a sequence (string, array) and using the stack to temporarily hold items while you make decisions based on the most recently added item.

A quintessential pattern is checking for valid, nested pairs—like parentheses, tags, or brackets. This directly applies to validating structured data formats (e.g., simplified JSON, XML). The algorithm pushes opening symbols onto the stack and pops them off when a matching closing symbol is found.

def is_valid(s: str) -> bool:
    stack = []
    mapping = {')': '(', '}': '{', ']': '['}
    for char in s:
        if char in mapping:  # Closing bracket
            top_element = stack.pop() if stack else '#'
            if mapping[char] != top_element:
                return False
        else:  # Opening bracket
            stack.append(char)
    return not stack  # Valid if stack is empty

Recommended Practice Order

Build competence progressively:

1. Fundamentals: Implement a stack using an array/list, then solve "Valid Parentheses" and "Min Stack."
2. Core Patterns: Practice "Next Greater Element," "Evaluate Reverse Polish Notation," and "Daily Temperatures." These solidify the traversal-and-stack pattern.
3. Simulation: Try "Simplify Path" and "Remove All Adjacent Duplicates In String" to handle stateful processing.
4. Hybrid Challenges: Finally, tackle problems like "Largest Rectangle in Histogram" or "Asteroid Collision," which combine stacks with other algorithmic thinking.

Practice Stack at Expedia