Hash-map lookups, from Two Sum to O(n)

The most common junior-interview footgun is a nested loop where a hash-map lookup would do. This track pulls together the problems where the same pattern — remember what you've seen so far, check the complement — collapses O(n²) code into a single pass.

You'll leave with a reflexive habit: when the naive solution asks the same lookup question repeatedly, reach for a hash map before you reach for a second loop.

Follow the order

1. 01

   problem

   Two Sum

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

   The canonical example. Understand why the hash map replaces the inner loop, not just how.

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2. 02

   article

   Why we run every code submission in a Firecracker microVM

   Every submission on ls-judge runs in its own Firecracker microVM — the same primitive AWS Lambda uses. Here is why we picked the hardware-isolation route over containers, and what it cost us to get there.

   Detour — how ls-judge itself isolates every submission. Skip if you're here for the pattern only.

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