I have been learning c/c++ for many months and I am still beginner, but I want to go to more advanced level.
My question is how do I approach to the next level? I have been doing same programs with different approaches and logic and method. I want to know which books should I refer also I want to know where can I find intermediate level programs to read and learn more.
I am in the same boat as you are. Beginner, attempting to jump to intermediate and beyond I guess. You have to take these generalisations with a pinch of salt though. What exactly do C++ programmers assume when they refer to terms "beginner", "intermediate" and so on is anybody's guess.
It is all rather too subjective IMHO.
Having said all that, when I started out with C++, some time towards the end of last year, I found that Stephen Prata's book, C++ Primer Plus 6th Edition was helpful and detailed enough to get a good understanding. Especially chapters 10 to 16. Although a bit dated now, especially in view of C++17 having recently been introduced, it is still worth reading and practising using the exercises provided at the end of each chapter.
The other book, regardless of what level, which is a "must" read in the C++ world is "Bjarne Stroustrup - The C++ Programming Language - 2013".
There are also several books that cover the STL, which you'll need to tackle, but only once you're somewhat more comfortable with the language.
@boost Lexical Cast..
I am reading books on STL (C++ standard Library by Nicolai Josuttis), I am getting myself familiar with standard c++ library, but I am going with slow pace. reading and coding slowly.
coding the same program with different approach etc and asking questions on this forum.
That's good. The standard library can be useful. Prevents a lot of reinventing. http://en.cppreference.com/w/ is a good reference for the C++ standard library.
The next level could be writing a useful program that you will actually use. Or an entertaining one (like games). I prefer to create command line tools. For example, creating a command line tool to find files with something like boost::filesystem. Or GUI programs. E.g. Timer and Stopwatch, calculator, etc.