Stack Questions at Zenefits: What to Expect

Stack questions appear in about 14% of Zenefits' technical interview problems (3 out of 21). While not the most frequent topic, they are a consistent and critical component. Successfully solving stack problems demonstrates your ability to manage state, handle nested structures, and process data in a Last-In-First-Out (LIFO) order—skills directly applicable to parsing configurations, evaluating expressions, or managing UI state histories in Zenefits' HR and benefits platform. Missing a stack problem can be a significant disadvantage.

What to Expect — Types of Problems

At Zenefits, stack questions typically fall into two categories. First, parsing and validation problems, such as checking for balanced parentheses, HTML tags, or a valid sequence of operations. These test your attention to edge cases. Second, monotonic stack problems, where you maintain a stack in increasing or decreasing order to efficiently solve problems like "next greater element" or "stock span." These assess your ability to optimize for time complexity by eliminating unnecessary comparisons. Expect the problem statement to have a real-world flavor, like validating a user-entered formula or calculating the next available appointment slot, but the core will be a classic stack algorithm.

How to Prepare — Study Tips with One Code Example

Focus on mastering the core patterns, not memorizing solutions. Implement stacks using the standard library (list in Python, Array in JavaScript, Deque in Java). Practice drawing the stack state at each step for a few examples. A fundamental pattern is using a stack to track openers for closers. Here is the classic valid parentheses solution:

def is_valid(s: str) -> bool:
    stack = []
    mapping = {')': '(', '}': '{', ']': '['}

    for char in s:
        if char in mapping:  # It's a closing bracket
            top_element = stack.pop() if stack else '#'
            if mapping[char] != top_element:
                return False
        else:  # It's an opening bracket
            stack.append(char)

    return not stack  # Valid if stack is empty

Recommended Practice Order

Build competency in this sequence: 1) Fundamentals: Valid Parentheses, Min Stack. 2) Evaluation: Reverse Polish Notation, Basic Calculator. 3) Monotonic Stack: Next Greater Element, Daily Temperatures. 4) Hybrid Problems: Largest Rectangle in Histogram (uses both a stack and an array). Solve each problem from scratch, then analyze time/space complexity. Timebox yourself to 25 minutes per problem to simulate interview pressure.

Practice Stack at Zenefits