Introduction

Whether you’re a student preparing for coding interviews or a software engineer aiming to sharpen your problem-solving skills, Data Structures & Algorithms (DSA) are your secret weapon. Coupled with LeetCode practice, mastering DSA can dramatically improve your ability to tackle technical interviews, optimize code, and solve complex problems efficiently.

This guide breaks down essential DSA concepts into easy-to-understand lessons and pairs them with carefully selected LeetCode problems. Each chapter includes theory, example problems, solutions in JavaScript, and explanations, giving you both knowledge and practice in one place.

By the end of this blog post, you’ll have a roadmap for learning DSA and practical experience solving problems that top tech companies love to ask.

Problem 1: Two Sum (Easy)

LeetCode #1

Description:
Given an array nums and an integer target, return indices of two numbers that add up to target.

Solution (JavaScript):

Explanation:

• Use a hash map to store numbers and their indices.

• Check if target - nums[i] exists in the map for O(1) lookup.

• Time Complexity: O(n)

• Space Complexity: O(n)

Problem 2: Maximum Subarray (Medium)

LeetCode #53

Description:
Find the contiguous subarray with the largest sum.

Solution (Kadane’s Algorithm):

Explanation:

• At each step, decide whether to continue the current subarray or start a new one.

• Time Complexity: O(n)

• Space Complexity: O(1)

Problem 3: Move Zeroes (Easy)

LeetCode #283

Description:
Move all zeros to the end of the array while maintaining the order of non-zero elements.

Solution:

Explanation:

• Use two pointers: one iterates, the other tracks the last non-zero element.

• Time Complexity: O(n)

• Space Complexity: O(1)

Problem 4: Best Time to Buy and Sell Stock (Easy)

LeetCode #121

Description:
Find the maximum profit from a single buy and sell.

Solution:

Explanation:

• Track the minimum price so far and maximum profit.

• Time Complexity: O(n)

• Space Complexity: O(1)

Problem 5: Product of Array Except Self (Medium)

LeetCode #238

Description:
Return an array such that each element is the product of all other elements.

Solution:

Explanation:

• Compute left and right products in two passes.

• Time Complexity: O(n)

• Space Complexity: O(1) extra (excluding output)

Chapter 1 Summary

• Arrays are essential for most algorithm problems.

• Patterns to master: two pointers, sliding window, prefix/suffix, hash maps, Kadane’s algorithm.

• Practice the 5 problems above before moving on to Strings.