Stack Questions at SAP: What to Expect

Stack questions appear in about 9% of SAP's technical interview problems (4 out of 45). While this may seem like a small portion, these questions are highly predictable and test a candidate's ability to manage data with Last-In-First-Out (LIFO) logic—a pattern directly applicable to software scenarios involving parsing, state tracking, and undo operations. Mastering stack fundamentals is a high-return investment for SAP interviews, as these problems are often used to cleanly filter candidates on core algorithmic thinking.

What to Expect — Types of Problems

SAP's stack questions typically fall into three categories, focusing on clean implementation rather than obscure variations.

1. Parentheses & Syntax Validation: The most common type. You'll be asked to check for balanced brackets ((), {}, []) in a string of code or expression. This directly tests using a stack to track opening symbols and match them with closings.
2. Stack-Based Calculations: Problems like evaluating a postfix (Reverse Polish Notation) expression or a basic calculator ("3+2*2"). These require using stacks to hold numbers and defer operations based on precedence.
3. Adjacent Value Removal: Problems where you must iteratively remove adjacent duplicate characters or values from a sequence, like repeatedly removing "aa" from "abbaca" to get "ca". A stack is the natural, efficient structure for this "collision" detection.

You will not need to implement complex stack variations from scratch. Focus on using the standard language library (list in Python, Array in JavaScript, Deque or Stack in Java) to solve these classical problems.

How to Prepare — Study Tips with One Code Example

Prepare by mastering the core pattern: iterate through input, use the stack to remember context, and pop when a condition is met. Practice writing bug-free code for the three problem types above. Internalize this pattern with the classic parentheses validation problem.

Key Pattern Example: Valid Parentheses Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if open brackets are closed by the same type and in the correct order.

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 competence sequentially:

1. Valid Parentheses (above) — The absolute fundamental.
2. Remove All Adjacent Duplicates In String — Apply the same stack pattern to a different condition.
3. Evaluate Reverse Polish Notation — Learn to use a stack for calculation.
4. Basic Calculator II — A more advanced integration of stacks and operation precedence.

Solve each problem until you can code it correctly on the first try without referencing solutions. This builds the muscle memory needed for an interview setting.

Practice Stack at SAP