Leetcode 1 | Two Sum Explained in Bangla | Python | Neetcode 150 #3

Two Sum (LeetCode 1) — the most famous coding interview question — solved step by step in Bangla, the way you should explain it in a real interview. We go from the O(N²) brute force to the optimal O(N) one-pass Hash Map solution using the complement lookup pattern: for every number, check if (target − number) has already been seen. What you'll learn: ✅ Clarifying questions & constraints (sorted? negatives? same element twice?) ✅ Edge cases: negatives, duplicate values, same index rule ✅ Brute force with nested loops → O(N²) ✅ Thinking in reverse: the complement (target − num) ✅ Why Hash Map, not Hash Set or Two Pointers ✅ Clean Python code + line-by-line dry run of 4 test cases ✅ Time O(N) & Space O(N) analysis ⏱️ Chapters 0:00 Intro – NeetCode 150 #3 0:19 Problem statement (LeetCode 1) 1:06 Step 1 – Clarifying questions & constraints 1:44 Step 2 – Edge cases & test cases 3:20 Step 3 – Brute force: check every pair 3:57 Brute force walkthrough – O(N²) 5:05 Step 4 – Think in reverse: the complement 6:14 Why Hash Map (not Hash Set / Two Pointers) 6:59 Code – one-pass Hash Map 7:37 Dry run – 4 test cases 10:31 Time & space complexity 10:58 Pattern takeaway 🧩 Pattern: Hash Map Complement Lookup Use it when you need a matching value for each element AND the index of an earlier element. 📚 NeetCode 150 series (Bangla) #1 Contains Duplicate – Hash Set #2 Valid Anagram – Frequency Counting (Hash Map) #3 Two Sum – Complement Lookup (Hash Map) 🔗 Problem: https://leetcode.com/problems/two-sum/ #TwoSum #NeetCode150 #LeetCode #DSA #CodingInterview #Bangla