50,000,000+ submissions. The most-attempted problem on LeetCode. And most of them solve it the slow way.

This is the Two Sum problem broken down four ways — brute force, two-pass hash map, one-pass hash map, and two-pointer — with full JavaScript code, complexity analysis, and the one trap interviewers love to set with the two-pointer approach.

By the end you'll know exactly which solution to write, when to write it, and how to talk through it without sounding like you memorized a LeetCode answer.

CHAPTERS
00:00 You've solved Two Sum wrong
00:25 The problem + examples + constraints
01:15 Brute force O(n²) — and why it scales like garbage
02:30 The insight: stop searching, start remembering
03:00 Two-pass hash map — O(n) time, O(n) space
03:45 One-pass hash map — the version interviewers want
04:50 Two-pointer (sorted) — the index trap
06:00 Comparison table: time, space, original indices
07:00 When to bring up which approach
07:25 The question for the comments

KEY TAKEAWAYS
· Brute force ships, but it's O(n²) — quadratic dies fast
· One-pass hash map is the optimal: O(n) time, O(n) space, 8 lines
· Two-pointer wins on the SORTED variant only — original indices die when you sort
· State the brute force first, out loud — interviewers want to see the path

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