Arrays, strings, and the two-pointer idea
By the end of today you can recognise when a problem over a sequence can be solved with two moving indices instead of nested loops, and turn an O(n squared) solution into an O(n) one.
YesterdayOn Day 13 you learned that a nested loop over n items costs roughly n squared. Today you learn the first standard way to avoid one.
TomorrowTomorrow, hash maps give you a different way to collapse the same kind of loop.
Why this matters
Two pointers is the first algorithm pattern most people learn, and the first time you see a nested loop collapse into a single pass. That collapse is the core move of most interview problems.
- Arrays and strings
- Two pointers
- Working in place
- Slicing cost
Learn it
70 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
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Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
3 hand-picked resources
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Build it
45 minWrite a palindrome check twice: once by reversing the string and comparing, once with two pointers moving inward and no extra allocation. Then solve 'find two numbers in a sorted list that sum to a target' with nested loops first, and again with two pointers. Time both pairs at 10,000 items.
Recall it
20 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
4 recall questions
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Rate it
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Mastery tracking
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Recap
- 01Arrays are contiguous, so indexing is instant and inserting is not
- 02Two pointers collapses many nested loops into one pass
- 03Strings are immutable, so repeated concatenation copies repeatedly
- 04Slicing copies, which matters inside a loop
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.