Stack Questions at Intuit: What to Expect

Stack questions appear in roughly 14% of Intuit's technical interview problems, making them a core data structure to master. For a company that builds complex financial and tax software like TurboTax and QuickBooks, evaluating expressions, parsing data, managing function calls, and handling nested structures are daily engineering tasks. The stack’s Last-In-First-Out (LIFO) principle is perfectly suited for these scenarios, which is why interviewers use these problems to assess your ability to model real-world system behavior and write clean, efficient code.

What to Expect — Types of Problems

At Intuit, stack questions typically fall into three categories:

1. Expression Evaluation: Problems involving parsing and calculating arithmetic expressions, often with parentheses and operator precedence. This directly relates to calculating fields in financial forms or tax logic.
2. Validation & Parsing: Checking for balanced parentheses, valid HTML/XML tags, or correct sequence order. This tests your ability to ensure data integrity and proper structure.
3. Next Greater Element & Monotonic Stack: Problems that require finding the next greater or smaller element in an array. These pattern-matching questions assess optimization skills for processing streams of data, like transaction histories.

Expect the problems to have a clear connection to Intuit's domain, such as evaluating a formula string or validating the nested structure of a document.

How to Prepare — Study Tips with One Code Example

Focus on understanding the core patterns, not just memorizing solutions. The most frequent pattern is using a stack to track openers (like (, [, {) and match them with closers. Always walk through edge cases: empty input, single characters, and already-balanced sequences.

Here is the classic valid parentheses problem, a fundamental pattern you must know:

def isValid(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

Recommended Practice Order

Build competency progressively:

1. Start with fundamental operations (valid parentheses, stack implementation).
2. Move to expression evaluation (postfix/infix conversion and calculation).
3. Tackle monotonic stack problems (next greater element, daily temperatures).
4. Finally, solve simulation problems that mimic real workflows, like a browser history manager or a document undo feature.

This order ensures you internalize the simple LIFO mechanics before applying them to complex, layered problems typical of Intuit's systems.

Practice Stack at Intuit