please reply fast
you can use bitwise operators. 
can you explain it because i don,t know about bitwise operator
You can use a recursive algorithm. You can take 2 cases :
1.The element is included in the subset
or
2.The element is not included in the subset.
You can refer to this stackoverflow answer for further reference and logic
i already seen this link but i m not comfortable in java please provide any c++ link
Just try and understand the logic and code on your own. The logic used is as follows:
The logic used is that the for each iteration the coder performs 2 recursive calls.
First he pushes the number into a stack(the number is a part of the subset) and is in the position say i. He then calls the function recursively to find the remaining i-1 numbers.
Then he pops the number from the answer stack (the number is not a part of the subset) and calls the function recursively again to find the remaining i elements.
Try to code with this logic.Refer to this Quora answer (though not exactly this question this is somewhat similar to this one and is in c++).
https://www.quora.com/What-is-the-recursive-solution-for-finding-all-subsets-of-a-given-array
If possible I will try to provide a C++ solution later in the day.
Please upvote if you like my answer.
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it will print subsets of all lengths, so before actually printing u need to check whether the number of “ON” bits is equal to k or not, if equal then only print.In addition to that it is faster then recursive algorithms cause you are operating with bitwise operations, without making any function calls.
Before opening this link : http://www.sanfoundry.com/c-program-generate-k-elements-subset/
try it yourself first. Happy coding