Especially for tackling the ones that come in ACM-ICPC. Which OJ has the best math(especially number theory) problems, that, will help me master it? Is Codeforces good for mastering the concepts?
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Especially for tackling the ones that come in ACM-ICPC. Which OJ has the best math(especially number theory) problems, that, will help me master it? Is Codeforces good for mastering the concepts?
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codeforces not bad


http://codeforces.me/problemset/tags/number%20theory
You can try SPOJ Classifier
Number theory problems
Highly recommend Project Euler. It is, basically, a maths problem-solving site, which has problems from various topics, and also requires programming skills. If your maths skills are somewhere close to Regional, maybe starting with 25% / 30% difficulty is not a bad idea. Of course there are some problems which are not really helpful, especially geometric simulation ones, which prefer to be solved with Mathematica, MatLab etc.- just skip them and move to other problems. The bad thing is, it has no official hints/ editorials, so just don't get stuck at a problem for too long ;)
On the other hand, Codeforces and TopCoder problems are also quite good, especially in probabilities. And they have editorials :)