LeetCode 128: Longest Consecutive Sequence | DSA Series Day 20

Welcome back to Day 20 of my Data Structures and Algorithms (DSA) series! Today, we are tackling one of the most frequently asked medium-level interview questions: LeetCode 128 - Longest Consecutive Sequence. In this video, we are given an unsorted array of integers and need to find the length of the longest consecutive elements sequence. The major catch? Our algorithm must run in strictly O(N) time complexity! This means we cannot simply sort the array (which would take O(N log N) time). I will walk you through how to use a Hash Set (std::unordered_set in C++) to achieve this O(N) runtime. I'll explain the crucial logic of identifying the "start" of a sequence and counting upwards efficiently without redundant checks. Code Repository & Solutions: Get the full C++ source code for this problem and all my series solutions on my GitHub: https://github.com/SaptakBhattacharyy... Connect with Me: Track my progress and connect with me on LeetCode: https://leetcode.com/u/SaptakBhattach... Timestamps: 0:00 - Intro & Day 20 Welcome 0:45 - Understanding the Problem (LeetCode 128) 1:30 - Why Sorting Fails the O(N) Requirement 2:45 - The Hash Set Approach & Finding the "Sequence Start" 4:30 - Writing the C++ Code 6:45 - Time & Space Complexity Analysis (True O(N) Explained) 8:00 - Outro & What's Next for Day 21 Tags: #DSA #LeetCode #LeetCode128 #LongestConsecutiveSequence #HashSets #Arrays #DataStructures #Algorithms #CompetitiveProgramming #CodingInterview #SoftwareEngineering #C++