Stack Questions at Josh Technology: What to Expect

Stack questions appear in nearly 1 out of 6 problems at Josh Technology, making them a core component of their technical interviews. Mastering stacks is non-negotiable because they test a candidate's ability to manage data in a Last-In-First-Out (LIFO) order, which is fundamental to parsing, reversing, and validating sequences—skills directly applicable to real-world development tasks like syntax checking, expression evaluation, and undo/redo operations. A strong performance here signals you can handle state management and algorithmic thinking.

What to Expect — Types of Problems

Josh Technology's stack problems typically fall into three categories:

1. Parentheses & Expression Validation: Checking for balanced brackets or evaluating postfix expressions.
2. Next Greater/Smaller Element: Finding the next element meeting a condition in an array, a classic monotonic stack pattern.
3. Stack as a Tool in Larger Algorithms: Using a stack to simulate recursion, maintain a min/max, or as part of a DFS traversal.

Expect problems that are direct applications of these patterns, not obscure variations. The focus is on clean implementation and handling edge cases.

How to Prepare — Study Tips with One Code Example

Focus on understanding the core patterns, not memorizing solutions. Practice drawing the stack state for each input step. Implement each pattern from scratch multiple times. For example, the Next Greater Element pattern is essential. You traverse an array, using a stack to store indices of elements for which the next greater element hasn't been found yet, maintaining a decreasing order.

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

    for i in range(n):
        # While current element > element at stack's top index
        while stack and nums[i] > nums[stack[-1]]:
            idx = stack.pop()
            result[idx] = nums[i]
        stack.append(i)
    return result

# Example: [4, 5, 2, 10] -> [5, 10, 10, -1]

Recommended Practice Order

1. Fundamentals: Implement a stack class, balanced parentheses, and stack using queues.
2. Core Patterns: Next Greater Element, stock span problem, and celebrity problem.
3. Expression Evaluation: Infix to postfix conversion and postfix evaluation.
4. Advanced Integration: Largest rectangle in histogram, binary tree traversals using stack, and min stack.

Solve problems in this sequence to build competence incrementally. Time yourself to match interview conditions.

Practice Stack at Josh Technology