

Ask HN: website / mobile app for algorithms practice? - zxcvvcxz

I'm a casual Chess player. When I don't have time to play a full game, I like to open this app I have called tactics trainer that just loads a board and asks me to find the right sequence of moves. If I get it wrong, it shows me the answer right away. It lets me practice chess without the actual time commitment of a full game.<p>In a similar sense I'd love to know if there's an algorithm site (preferably an offline phone app!) that presents a problem, project Euler style, and then after you've given it sufficient thought you can click on a "solution" button which will have some text, maybe a diagram or whatever, explaining the solution.<p>Here I'll give you guys a little teaser:<p>Q: You have an array with N elements, each of which are integer numbers (from 1 to 1 million say). Each of the numbers is repeated twice except for one of the numbers, which occurs only once. Using constant-sized memory, find out which number is only repeated once.<p>A: I'm sure HN will solve this one.<p>So, have something good along these lines for my commute? Cheers.
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bcjordan
Code Puzzles [1] sounds like exactly what you're talking about for iOS, and
it's a really well put together app with very clear solution write-ups.
There's also Cracking the Coding Interview's app [2] which has mobile videos
for purchase.

On the non-mobile side, there's the classic Project Euler [3] and Puzzle Node
[4].

If you search around on Quizlet, you can find flash cards for just about any
CS topic, including programming interview topics [5]. There are some mobile
apps for Quizlet viewing as well.

[1] [https://itunes.apple.com/us/app/code-
puzzles/id566255734?mt=...](https://itunes.apple.com/us/app/code-
puzzles/id566255734?mt=8)

[2] [https://itunes.apple.com/us/app/cracking-coding-
interview/id...](https://itunes.apple.com/us/app/cracking-coding-
interview/id491856690?mt=8)

[3] <http://projecteuler.net/problems>

[4] <http://puzzlenode.com/>

[5] [http://quizlet.com/16481093/programming-interview-
questions-...](http://quizlet.com/16481093/programming-interview-questions-
flash-cards/)

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zxcvvcxz
Do you know any Android apps you could recommend? Code Puzzles looks amazing.

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victorhn
Xor all the elements, the result is the number you are looking for :)

