🔥 Ultimate Coding Interview Cheat Sheet (2025 Edition)

✅ 1. Data Structures
Key Concepts:
• Arrays/Lists
• Strings
• Hashmaps (Dicts)
• Stacks & Queues
• Linked Lists
• Trees (BST, Binary)
• Graphs
• Heaps

Practice Questions:
• Reverse a string or array
• Detect duplicates in an array
• Find missing number
• Implement stack using queue
• Traverse binary tree (Inorder, Preorder)

✅ 2. Algorithms
Key Concepts:
• Sorting (Quick, Merge, Bubble)
• Searching (Binary search)
• Recursion
• Backtracking
• Divide & Conquer
• Greedy
• Dynamic Programming

Practice Questions:
• Fibonacci with DP
• Merge sort implementation
• N-Queens Problem
• Knapsack problem
• Coin change

✅ 3. Problem Solving Patterns
Important Patterns:
• Two Pointers
• Sliding Window
• Fast & Slow Pointer
• Recursion + Memoization
• Prefix Sum
• Binary Search on answer

Practice Questions:
• Longest Substring Without Repeat
• Max Sum Subarray of Size K
• Linked list cycle detection
• Peak Element

✅ 4. System Design Basics
Key Concepts:
• Scalability, Load Balancing
• Caching (Redis)
• Rate Limiting
• APIs and Databases
• CAP Theorem
• Consistency vs Availability

Practice Projects:
• Design URL shortener
• Design Twitter feed
• Design chat system (e.g., WhatsApp)

✅ 5. OOP & Programming Basics
Key Concepts:
• Classes & Objects
• Inheritance, Polymorphism
• Encapsulation, Abstraction
• SOLID Principles

Practice Projects:
• Design a Library System
• Implement Parking Lot
• Bank Account Simulation

✅ 6. SQL & Database Concepts
Key Concepts:
• Joins (INNER, LEFT, RIGHT)
• GROUP BY, HAVING
• Subqueries
• Window Functions
• Indexing

Practice Queries:
• Get top 3 salaries
• Find duplicate emails
• Most frequent orders per user

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Here are some tricky🧩 SQL interview questions!

1. Find the second-highest salary in a table without using LIMIT or TOP.

2. Write a SQL query to find all employees who earn more than their managers.

3. Find the duplicate rows in a table without using GROUP BY.

4. Write a SQL query to find the top 10% of earners in a table.

5. Find the cumulative sum of a column in a table.

6. Write a SQL query to find all employees who have never taken a leave.

7. Find the difference between the current row and the next row in a table.

8. Write a SQL query to find all departments with more than one employee.

9. Find the maximum value of a column for each group without using GROUP BY.

10. Write a SQL query to find all employees who have taken more than 3 leaves in a month.

These questions are designed to test your SQL skills, including your ability to write efficient queries, think creatively, and solve complex problems.

Here are the answers to these questions:

1. SELECT MAX(salary) FROM table WHERE salary NOT IN (SELECT MAX(salary) FROM table)

2. SELECT e1.* FROM employees e1 JOIN employees e2 ON e1.manager_id = (link unavailable) WHERE e1.salary > e2.salary

3. SELECT * FROM table WHERE rowid IN (SELECT rowid FROM table GROUP BY column HAVING COUNT(*) > 1)

4. SELECT * FROM table WHERE salary > (SELECT PERCENTILE_CONT(0.9) WITHIN GROUP (ORDER BY salary) FROM table)

5. SELECT column, SUM(column) OVER (ORDER BY rowid) FROM table

6. SELECT * FROM employees WHERE id NOT IN (SELECT employee_id FROM leaves)

7. SELECT *, column - LEAD(column) OVER (ORDER BY rowid) FROM table

8. SELECT department FROM employees GROUP BY department HAVING COUNT(*) > 1

9. SELECT MAX(column) FROM table WHERE column NOT IN (SELECT MAX(column) FROM table GROUP BY group_column)