Two Sum | AlgoChronicle

Step 1

Know these before you start

Tick what you already know

Concept video

Step 2

Try it on paper

5–10 minutes, no code yet

ProblemProblem statementGiven an array of integers nums and an integer target, return the indices of the two numbers that add up to target.Read

Problem Statement

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

Re-solved 2026-10-09 as Day 1 of the foundations rebuild (originally logged as day001).

Input / Output

Input:  nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: nums[0] + nums[1] = 2 + 7 = 9

Understanding the Problem

For each x, the partner you need is target - x
A hash map remembers every value seen so far and its index → O(1) lookups
Check the complement BEFORE inserting the current element — this avoids pairing an element with itself (e.g. nums = [3,3], target = 6)

Edge Cases

Duplicate values ([3,3]) → check-before-insert handles it
Exactly one valid answer is guaranteed → no missing-return worry

Step 6

Get it accepted

All test cases pass

Step 7

Review it so it sticks

Re-solve from memory after

Solve it yourself before you look

Reading the solution first feels like progress, but it makes the next similar problem — and the interview version — much harder, because you skipped the part where you figure it out. Give it an honest 20–30 minutes. Stuck? Re-read the pattern, watch the concept video, or try the brute force first.

Hidden: approach · solution code · optimal solution

Revising this one?

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→Say the pattern out loud, not just the code
→Explain it like you would to a friend
→Try it again in a different language