LeetCode Problem Solving: A Guide for Aspiring Coders.
Introduction
Looking for a job in tech--either at a major tech company or a fast growing startup--getting good at practicing problem solving on LeetCode can give you the edge you need. LeetCode is one of the major websites to practice coding problems that are in the illusion of technical interview questions at the workplace. However, given thousands of problems to select from, it is a daunting task to know where to begin, how to practice in a meaningful way, and how to learn and grow over time.
In this guide, we’ll walk you through a step-by-step approach to problem-solving on LeetCode, including choosing the right problems, honing your skills, and developing a strategy that will help you succeed.
Why LeetCode Problem Solving Matters
LeetCode problems address an eclectic set of data structures, algorithms, and programming foundations that are relevant to technical interviews. If not to get that right entry-on-the-coding-interview-door-buster, solving problems on LeetCode exposes your thinking in logical order, problem body sub-assembling, and efficient coding. All of these are abilities that can serve you well throughout your career.
Step 1: Get Comfortable with the Basics
Try to be sure with the basics of data structures and algorithms before tackling a complex one. Here are some of the essentials:
- Data Structures: Arrays, Linked Lists, Stacks, Queues, Trees, Graphs, Heaps
- Algorithms: Sorting, Searching, Recursion, Dynamic Programming, Greedy Algorithms
- Complexity Analysis: Understanding Big-O notation to analyze time and space complexity
GeeksforGeeks, Khan Academy, or YouTube tutorials helpful.
Step 2: Choose the Right Problems
Due to tens of thousands of problems on LeetCode, deciding which problems to practice can seem overwhelming. Here’s a recommended approach to problem selection:
1. Start with Easy Problems: Build your confidence by solving easier problems first. Search for tags "Easy" and solve problems related to basic data structures (arrays, string, hash maps).
2. Progress to Medium-Level Problems: Once you’re comfortable, begin solving "Medium" level problems. There exist similar challenging problems that are more like interview questions and are aimed at helping you develop critical problem solving skills.
3. Tackle Hard Problems When Ready: Don’t rush to hard problems too quickly. Medium-level problems instead and aim to solve them reliably in a time span from 20–40 minutes. Finally, gradually transition to hard problems once you become comfortable.
Tip: LeetCode categorizes problems by topics such as "Arrays," "Strings," "Linked Lists," and "Dynamic Programming.". That allows, at the same time, to just aim certain specific locations where you need practice.
Suggested Easy Problems to Start With:
- Two Sum (Array/Hashing)
- Valid Parentheses (Stack)
- Reverse Linked List (Linked List)
Suggested Medium Problems:
- Longest Substring Without Repeating Characters (String/Hash Map)
- Combination Sum (Backtracking)
- Binary Tree Level Order Traversal (Tree/BFS)
Step 3: Develop a Consistent Practice Routine
Solving problems on LeetCode requires regular practice to improve. Make an effort to devote a minimal amount of time every day to LeetCode instead of studying everything at the same time, e.g., on a weekend. Here’s a suggested weekly practice routine:
- Day 1–2: Solve two easy problems to warm up. Prioritize a complete problem understanding and clean code.
- Day 3–5: Solve one or two medium problems each day. Work on different topics each day to cover a variety of data structures and algorithms.
- Day 6: Review the problems you solved. Identify areas in which to refine your solutions, in particular as regards to time and space complexity.
- Day 7: Try a new or difficult problem to challenge yourself and apply what you’ve learned throughout the week.
Tip: Practicing a little each day helps you build muscle memory for coding and problem-solving techniques, making it easier to recall them during interviews.
Step 4: Break Down Problems with a Problem-Solving Framework
If you work on a fresh problem on LeetCode, having a framework to guide you is useful. Here’s a step-by-step approach:
1. Understand the Problem:
- Read the problem statement carefully. Identify the input and output formats.
- Ask yourself: What is the problem asking for? Are there any edge cases?
Write notes or draw examples in order to have an understanding of it.
2. Identify the Data Structure or Algorithm Needed:
- LeetCode problems often hint at which data structure or algorithm to use (e.g., "find the shortest path" suggests using a BFS or Dijkstra's algorithm).
- Think about similar problems you’ve solved to see if they share common patterns.
3. Plan Your Solution:
- Write down the steps your algorithm will follow. This can be a simple outline or pseudocode.
- Break the problem into smaller sub-problems if possible. For example, dividing a sorting problem into "cut" and "conquer" steps.
4. Implement the Solution:
- Start coding your plan. Don’t worry about perfection on the first try—focus on translating your plan into code.
- Use meaningful variable names, and try to keep your code clean and readable.
5. Test and Optimize:
Test your solution with normal cases and edge cases. Consider cases with empty inputs, large inputs, and boundary conditions.
For example, if it works, let's think of a more efficient solution for the problem.
Determine the time and space complexity and if it is not optimal, try to optimize them.
Step 5: Review Solutions and Learn from Mistakes
Comparing your own code to other versions is one of the most effective education means. LeetCode provides a useful Discussion section, in which you can observe ways in which others solved the problem.
1. Understand Your Mistakes: If you struggled with a problem, try to understand why. Were you unsure of the data structure? Did you struggle with edge cases? Identifying these patterns can help you improve.
2. Learn New Techniques: Other solutions discussed in the Discussion section may also teach you novel methods, algorithms, or optimizations that you did not even think about before.
3. Optimize Your Approach: If you find a better solution, refactor your code to implement the improvements.
Pro Tip: Record or maintain a notebook or document of solutions to problems. Include insights, common mistakes, and patterns you’ve identified. This will be a useful tool for future application and learning.
Step 6: Focus on Problem Patterns and Techniques
Although LeetCode contains thousands of distinct problems, most can be expressed in common terms, or using common methods. Recognize these patterns will allow you to get a solution faster. Here are some common patterns:
- Two Pointers: Used in problems involving arrays, strings, or linked lists where elements need to be compared or summed. (e.g., "Two Sum," "Valid Palindrome")
- Sliding Window: Most effective when applied to problems involving subarrays or substrings in which you must identify maxima, minima, or other conditions. (e.g., "Longest Substring Without Repeating Characters")
- Binary Search: Often used in sorted arrays or problems involving conditions. (e.g., "Search in Rotated Sorted Array")
- Backtracking: For problems that have every possible solution or pattern that can be selected, such as generating subsets or solving puzzles. (e.g., "Permutations," "Combination Sum")
- Dynamic Programming: Used frequently to solve the optimal substructure and overlapping subproblems, e.g., calculating a maximum profit or a minimum cost. (e.g., "Climbing Stairs," "House Robber")
Step 7: Take Notes and Create a Personal Cheat Sheet
The more problems you solve the more these patterns and techniques you will recognise. To make your study process easier, create a cheat sheet with key problem types, common techniques, and notes on time complexity.
Organize your cheat sheet by categories such as:
- Arrays
- Strings
- Linked Lists
- Trees/Graphs
- Dynamic Programming
- Backtracking
Provide a quick reference to your solution, practical edge cases, and approaches to optimization.
Step 8: Don’t Just Memorize—Understand
The technical interview is not complete unless a candidate knows, remembers a solution, but more importantly, can articulate the thinking and reasoning behind it. To gain a deep understanding of each solution, as opposed to rote memorization of procedures. This way, even if a problem changes slightly, you’ll be able to adapt your approach.
Final Thoughts
Developing the ability to solve LeetCode problems demands time, self-discipline, and a structured methodology. Just don't give up feeling challenged at the beginning; solving every problem makes progress the easier. By continued practice, pattern recognition, and a pinch of patience, the quality of your problem-solving ability will greatly improve.
