Two Sum

Two Sum — think like a teacher, not a memorizer

You get an array of integers and a target. Return indices of two numbers that add to target.

Wrong first instinct

Brute force: try every pair. Works, but O(n²). Interviews want the O(n) story.

The move

As you scan left→right, ask: "Have I already seen target - nums[i]?"

Keep a hash map: value → index.

Dry run

nums = [2,7,11,15], target = 9

• i=0, need 7, map empty → store 2→0
• i=1, need 2, map has 2 at 0 → return [0,1]

You already solved this on LeetCode. Rewrite it cleanly here without peeking.

Tip: Open the Animated Classroom for Two Sum — watch the map fill before you code.