In cases where we need to store 10^7 or more integers what is the best method for storing and accessing these data.Vector is one of the solution but is that the fastest because when we access by index ([]) I read there are some overhead. So is there any better method?

I tried to solve this problem http://mirror.codeforces.com/problemset/problem/459/C . After some time I checked out the editorial and i understood the basic idea but i cant understand how the sequence is generated.

http://mirror.codeforces.com/contest/459/submission/7495236 is the solution given in the editorial.

I am having a tough time in finding a way to practice . I know only the basic algorithms so i am thinking if i should learn some of the famous algorithms and practice problems related to that from ahmed-aly website or do random problems from oj. I am asking this because when i try to solve a problem i would spend some time on it then come to know that it required algorithm which i don't know and this pattern continues for most of the problems.But in some forums they say solving by topic is a bad way of practice.So i am hopeless here.

What strategy do u use for practicing ?

PS: I am a beginner who can solve A and B problems in cf contest and some times C.

I am new to this and I am learning dp. It's tough for me approach a dp problem but when i see the solution i get the idea and why they are doing it but iam not able to come up with it on my own.Can anyone tell me how you practiced dp like did u practice problems topic wise (dp in this case) or practice random problems.I am confused because when u practice by topic u will think in that way only and i dont think that's a good idea.If you also found any good materials for dp can u share it?

PS: Thanks for reading the post and Sorry for my bad English