W Code - Pattern-Based DSA Learning Platform

Bhanu Bisht

Template

Stack Patterns Template: Pattern, Code & Cheat Sheet

The Stack Patterns pattern is one of the most frequently tested coding interview patterns. LIFO-based solutions for parsing, matching, and evaluation. This template gives you a reusable code skeleton, pseudocode, and implementation in multiple languages so you can solve 11+ problems using this single mental model.

Difficulty: Medium | Time Complexity: O(n) | Space Complexity: O(n)

When to Use This Template

Use the Stack Patterns template when you see these signals in a problem:

1.Parentheses matching — Problems asking to parentheses matching

2.Expression evaluation — Problems asking to expression evaluation

3.Next greater element — Problems asking to next greater element

Prerequisites: Arrays basics

Problem count on W Code: 11 problems across Easy, Medium, and Hard difficulty levels.

If the problem does not match these signals, consider alternative patterns.

Pseudocode Template

function stack_patternsSolve(input):
    // Initialize data structures
    result = initial_value
    
    // Core logic for Stack Patterns
    for each element in input:
        process(element)
        update(result)
    
    return result

Python Implementation

python
def solve(input_data):
    """Stack Patterns solution template."""
    result = []
    # Implement stack patterns logic here
    for item in input_data:
        # Process each item
        result.append(item)
    return result

Java Implementation

java
public Object solve(Object[] input) {
    // Stack Patterns template
    // Implement core logic here
    return null;
}

C++ Implementation

cpp
auto solve(vector<int>& input) {
    // Stack Patterns template
    // Implement core logic
    return result;
}

Variations & Adaptations

The Stack Patterns pattern has several variations you should master:

Variation 1: Parentheses Matching

This variation is useful when the problem specifically requires parentheses matching. Adapt the main template by modifying the core loop/recursion logic accordingly.

Variation 2: Monotonic Stack

This variation is useful when the problem specifically requires monotonic stack. Adapt the main template by modifying the core loop/recursion logic accordingly.

Variation 3: Expression Evaluation

This variation is useful when the problem specifically requires expression evaluation. Adapt the main template by modifying the core loop/recursion logic accordingly.

Variation 4: Next Greater Element

This variation is useful when the problem specifically requires next greater element. Adapt the main template by modifying the core loop/recursion logic accordingly.

Common Mistakes & Edge Cases

When implementing Stack Patterns, watch out for:

1.Off-by-one errors — Carefully handle boundary conditions in loops

2.Missing base case — Ensure recursion terminates properly

3.Integer overflow — Use appropriate data types for large inputs

4.Empty input — Handle null/empty arrays, strings, or graphs

5.Single element — Test with minimal inputs

Edge cases to always test:

•Empty input → expected output

•Single element → correct result

•All same elements → no infinite loop

•Maximum constraints → within time limit

Step-by-Step Problem Solving Guide

1.Identify the pattern: Does the problem involve Parentheses matching, Expression evaluation, Next greater element?

2.Choose the template: Select the variation from above that best fits

3.Define variables: Map problem-specific entities to template variables

4.Implement the core logic: Fill in the template skeleton

5.Handle edge cases: Add boundary checks

6.Verify with examples: Trace through sample inputs

7.Analyze complexity: Confirm O(n) time and O(n) space

Frequently Asked Questions

What problems can I solve with the Stack Patterns template?

The Stack Patterns template applies to 11+ problems including: Parentheses matching, Expression evaluation, Next greater element. Look for problems that involve lifo-based solutions for parsing, matching, and evaluation..

What is the time complexity of Stack Patterns?

The typical time complexity is O(n) with space complexity of O(n). Some variations may differ.

What should I learn before Stack Patterns?

Prerequisites include: Arrays basics. Make sure you are comfortable with these concepts first.

How do I recognize a Stack Patterns problem in an interview?

Key signals: the problem involves parentheses matching; the problem involves expression evaluation; the problem involves next greater element. Practice identifying these cues by solving at least 10 problems using this pattern.

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