Where to Start with LeetCode: Complete Beginner Guide to Coding Interview Preparation

Introduction

If you're asking "where to start with LeetCode?", you're not alone. LeetCode can be overwhelming for beginners, with over 2,000 problems covering everything from arrays to advanced dynamic programming. This guide will help you navigate your LeetCode journey from absolute beginner to interview-ready.

Why LeetCode Matters

LeetCode is the gold standard for coding interview preparation. Top tech companies (FAANG, startups, and more) use LeetCode-style problems to assess candidates. Mastering LeetCode helps you:

• Solve problems efficiently under pressure
• Think algorithmically
• Write clean, optimized code
• Prepare for technical interviews
• Build confidence in problem-solving

Understanding LeetCode Difficulty Levels

Easy Problems

• Target: Beginners, warm-ups
• Time: 10-20 minutes
• Focus: Basic operations, simple logic
• Examples: Two Sum, Reverse String, Valid Parentheses

Medium Problems

• Target: Most interview questions
• Time: 30-45 minutes
• Focus: Common patterns, algorithms
• Examples: Longest Substring, Merge Intervals, Binary Tree Level Order

Hard Problems

• Target: Senior positions, advanced topics
• Time: 45-60+ minutes
• Focus: Complex algorithms, optimization
• Examples: Serialize Binary Tree, Trapping Rain Water, Word Ladder II

Where to Start: Your First 30 Days

Week 1: Foundation (Easy Problems)

Goal: Get comfortable with LeetCode interface and basic operations.

Problems to Solve (10-15 Easy problems):

1. Two Sum
2. Reverse Integer
3. Palindrome Number
4. Valid Parentheses
5. Merge Two Sorted Lists
6. Maximum Subarray
7. Climbing Stairs
8. Best Time to Buy and Sell Stock
9. Contains Duplicate
10. Valid Anagram

Focus Areas:

• Arrays and basic operations
• String manipulation
• Simple logic problems
• Understanding problem structure

Week 2: Arrays and Strings (Easy-Medium)

Goal: Master fundamental data structures.

Topics:

• Array manipulation
• String operations
• Two pointers technique
• Sliding window basics

Problems (15-20 problems):

1. Remove Duplicates from Sorted Array
2. Rotate Array
3. Move Zeroes
4. Reverse String
5. First Unique Character
6. Valid Palindrome
7. Longest Common Prefix
8. Group Anagrams
9. Product of Array Except Self
10. Container With Most Water

Week 3: Linked Lists and Stacks (Easy-Medium)

Goal: Understand pointer manipulation and stack operations.

Topics:

• Linked list traversal
• Stack operations
• Queue basics

Problems (15-20 problems):

1. Reverse Linked List
2. Merge Two Sorted Lists
3. Linked List Cycle
4. Remove Nth Node From End
5. Valid Parentheses
6. Min Stack
7. Daily Temperatures
8. Next Greater Element
9. Design Circular Queue
10. Design Stack

Week 4: Trees and Basic Algorithms (Easy-Medium)

Goal: Introduction to tree structures and recursion.

Topics:

• Tree traversal (DFS, BFS)
• Recursion basics
• Binary search

Problems (15-20 problems):

1. Maximum Depth of Binary Tree
2. Same Tree
3. Invert Binary Tree
4. Binary Tree Level Order Traversal
5. Validate Binary Search Tree
6. Binary Search
7. Search Insert Position
8. Find First and Last Position
9. Sqrt(x)
10. Search in Rotated Sorted Array

Essential Problem Patterns to Master

1. Two Pointers

When to use: Array/string problems, sorted arrays, palindromes

Example Problems:

• Two Sum (sorted array)
• Container With Most Water
• Trapping Rain Water
• Valid Palindrome
• Reverse String

Pattern:

left, right = 0, len(arr) - 1
while left < right:
    # Process arr[left] and arr[right]
    if condition:
        left += 1
    else:
        right -= 1

2. Sliding Window

When to use: Subarray/substring problems, fixed/variable window size

Example Problems:

• Maximum Sum Subarray of Size K
• Longest Substring Without Repeating Characters
• Minimum Window Substring
• Find All Anagrams in a String

Pattern:

left = 0
for right in range(len(arr)):
    # Expand window
    # Shrink window if needed
    while condition:
        left += 1

3. Hash Map/Set

When to use: Frequency counting, lookups, duplicates

Example Problems:

• Two Sum
• Group Anagrams
• Contains Duplicate
• Longest Consecutive Sequence

Pattern:

freq = {}
for num in nums:
    freq[num] = freq.get(num, 0) + 1

4. Binary Search

When to use: Sorted arrays, search optimization

Example Problems:

• Binary Search
• Search in Rotated Sorted Array
• Find Peak Element
• Search for a Range

Pattern:

left, right = 0, len(arr) - 1
while left <= right:
    mid = (left + right) // 2
    if arr[mid] == target:
        return mid
    elif arr[mid] < target:
        left = mid + 1
    else:
        right = mid - 1

5. DFS (Depth-First Search)

When to use: Trees, graphs, backtracking

Example Problems:

• Maximum Depth of Binary Tree
• Path Sum
• Number of Islands
• Generate Parentheses

Pattern:

def dfs(node):
    if not node:
        return
    # Process node
    dfs(node.left)
    dfs(node.right)

6. BFS (Breadth-First Search)

When to use: Level-order traversal, shortest path

Example Problems:

• Binary Tree Level Order Traversal
• Word Ladder
• Rotting Oranges
• Course Schedule

Pattern:

from collections import deque
queue = deque([start])
while queue:
    node = queue.popleft()
    # Process node
    for neighbor in neighbors:
        queue.append(neighbor)

Study Strategy: How to Approach Problems

Step 1: Understand the Problem (5 minutes)

• Read the problem carefully
• Identify inputs and outputs
• Understand constraints
• Ask clarifying questions (even if practicing alone)

Step 2: Think About Approach (10-15 minutes)

• Don't jump to coding immediately
• Consider multiple approaches
• Think about time/space complexity
• Start with brute force, then optimize

Step 3: Code the Solution (15-20 minutes)

• Write clean, readable code
• Use meaningful variable names
• Add comments for complex logic
• Handle edge cases

Step 4: Test Your Solution (5 minutes)

• Test with examples from problem
• Test edge cases (empty input, single element, etc.)
• Verify time/space complexity

Step 5: Review and Optimize (5-10 minutes)

• Compare with optimal solution
• Understand why it's better
• Learn the pattern
• Add to your notes

Recommended Problem Lists

LeetCode 75 (Blind 75)

Best for: Comprehensive coverage of common patterns

Categories:

• Array & Hashing (9 problems)
• Two Pointers (5 problems)
• Sliding Window (6 problems)
• Stack (7 problems)
• Binary Search (7 problems)
• Linked List (6 problems)
• Trees (15 problems)
• Tries (3 problems)
• Heap/Priority Queue (7 problems)
• Backtracking (9 problems)
• Graphs (13 problems)
• Advanced Graphs (6 problems)
• 1-D Dynamic Programming (12 problems)
• 2-D Dynamic Programming (11 problems)
• Greedy (8 problems)
• Intervals (6 problems)
• Math & Geometry (8 problems)
• Bit Manipulation (7 problems)

NeetCode 150

Best for: Structured learning path

• More comprehensive than Blind 75
• Organized by difficulty and topic
• Video explanations available
• Updated regularly

Grind 75

Best for: 8-week intensive preparation

• Curated 75 problems
• Weekly schedule provided
• Focus on most common interview questions

Tools and Resources

LeetCode Features

1. Problem Lists: Curated lists by company, topic, difficulty
2. Discuss Section: Learn from others' solutions
3. Solutions Tab: Official and community solutions
4. Playground: Test code without submitting
5. Mock Interviews: Timed practice sessions

External Resources

1. NeetCode YouTube: Video explanations
2. Tech Dummies Narendra L: Detailed problem walkthroughs
3. Back To Back SWE: Algorithm explanations
4. AlgoExpert: Video course with explanations
5. Grokking the Coding Interview: Pattern-based learning

Study Tools

1. Anki: Spaced repetition for patterns
2. Notion/Obsidian: Note-taking
3. LeetCode Extension: Track progress
4. Timer: Practice under time pressure

Common Mistakes to Avoid

❌ Mistake 1: Jumping to Hard Problems

Problem: Trying to solve Hard problems without foundation

Solution: Start with Easy, progress systematically

❌ Mistake 2: Only Reading Solutions

Problem: Not attempting problems yourself

Solution: Always try first, then review solutions

❌ Mistake 3: Not Understanding Patterns

Problem: Memorizing solutions instead of patterns

Solution: Focus on recognizing and applying patterns

❌ Mistake 4: Not Practicing Regularly

Problem: Cramming before interviews

Solution: Consistent daily practice (even 30 minutes)

❌ Mistake 5: Ignoring Time Complexity

Problem: Not analyzing or optimizing solutions

Solution: Always consider time/space complexity

Tracking Your Progress

Metrics to Track

• Problems solved: Total count
• By difficulty: Easy/Medium/Hard breakdown
• By topic: Arrays, Trees, DP, etc.
• Success rate: Problems solved on first try
• Time per problem: Average solving time

Weekly Goals

• Week 1-2: 10-15 Easy problems
• Week 3-4: 20-25 Easy-Medium problems
• Month 2: 30-40 Medium problems
• Month 3+: Focus on Medium, some Hard

Interview Preparation Timeline

3 Months Before Interview

• Month 1: Foundation (Easy problems, learn patterns)
• Month 2: Medium problems, mock interviews
• Month 3: Hard problems, company-specific practice

1 Month Before Interview

• Focus on company-tagged problems
• Mock interviews (2-3 per week)
• Review common patterns
• Practice explaining solutions

1 Week Before Interview

• Review notes and patterns
• Practice easy problems (build confidence)
• Mock interviews
• Rest and prepare mentally

Conclusion

Where to start with LeetCode is a common question, and the answer is: start simple, be consistent, and focus on patterns. Remember:

1. Start with Easy problems: Build confidence and familiarity
2. Learn patterns, not solutions: Understand when to use each approach
3. Practice consistently: Daily practice beats cramming
4. Track your progress: Monitor improvement
5. Use resources wisely: Combine LeetCode with explanations
6. Mock interviews: Practice under pressure

LeetCode mastery is a marathon, not a sprint. With consistent practice and the right strategy, you'll be interview-ready in 2-3 months. Start today, solve your first problem, and build momentum!

Your first step: Go to LeetCode, create an account, and solve "Two Sum" (Easy). You've got this!