Stack Questions at Anduril: What to Expect

Stack questions appear in about 12% of Anduril's technical interviews (5 out of 43 total problems). This isn't by accident. Anduril builds complex defense technology systems—think sensor fusion, autonomous platforms, and command-and-control software. These systems often involve processing nested data structures, managing state machines, parsing configuration or log files, and handling undo/redo operations. The stack's Last-In-First-Out (LIFO) property is perfect for tracking state, reversing sequences, and ensuring proper order of operations, making it a fundamental data structure for their engineering challenges. Mastering stack patterns is essential for demonstrating you can think clearly about control flow and state management in resource-constrained, real-time environments.

What to Expect — Types of Problems

Anduril's stack problems typically fall into two practical categories. First, parsing and validation questions are common. You'll likely encounter problems involving balanced parentheses, HTML/XML tag validation, or evaluating arithmetic expressions (like Reverse Polish Notation). These test your ability to process sequential data and enforce rules, mirroring real-world tasks like validating configuration files or parsing sensor data streams.

Second, expect state tracking and simulation problems. These often involve scenarios like simulating a browser's back/forward history, managing function calls in an interpreter, or finding the next greater element in an array. These problems assess how you manage layered or temporal state, a critical skill when working with autonomous system behaviors or multi-stage processes.

The problems are designed to be solved optimally with a stack; brute-force solutions will likely fail on larger test cases. Focus on recognizing the core need to track a sequence where you only care about the most recent, or "top," element that matches some condition.

How to Prepare — Study Tips with One Code Example

Start by deeply understanding the core LIFO operations: push, pop, peek, and isEmpty. Then, practice identifying the stack "trigger"—the moment in a problem where you must compare the current element with a previous one. A classic pattern is using a stack to find the next greater element for each item in an array. You iterate through the array, using the stack to hold indices of elements for which we haven't yet found a greater element. When you find a value greater than the element at the index stored at the stack's top, you've found your answer for that previous element.

def nextGreaterElement(nums):
    result = [-1] * len(nums)
    stack = []  # stores indices

    for i, num in enumerate(nums):
        # While stack is not empty and current number > number at stack's top index
        while stack and nums[stack[-1]] < num:
            idx = stack.pop()
            result[idx] = num
        stack.append(i)
    return result

Recommended Practice Order

Build competency progressively. First, master the fundamentals: implement a stack from scratch, then solve classic problems like valid parentheses and min stack. Second, tackle parsing problems: evaluate Reverse Polish Notation and basic calculator problems. Third, move to array-based stack problems: practice next greater element, daily temperatures, and stock span problems. Finally, simulate real-world systems: practice browser history simulation and asteroid collision problems. This order builds from simple LIFO mechanics to complex state management.

Practice Stack at Anduril