Some one else give answer and user accept it then your all handwork will be waste.
Not true! Just because a question has an accepted answer doesn't mean that any more effort spent on the question is wasted. Remember, this site is not just for the moment; future users come and see what solutions could help them.
Most of the time when I'm searching for a solution to an issue I'm having, it's been posted on Stack Overflow some one or two years ago. And that is precisely what the Q&A format is. If everyone had the mentality that "oh, this one has an accepted answer, I don't need to work on it", who knows what sort of trash would be around here. An accepted answer doesn't mean it is the BEST answer, it simply means it was the answer that (at that moment) the original poster felt best answered their problem.
Many times I give a concise answer that does answer the question. And for some users, that is plenty enough information for them to solve the issue on their own. Meanwhile, I can work on a more detailed answer, including code snippets and links to external resources that better explains the solution for users that might not quite grasp the concise answer.
If you've ever had an issue where you spent a large amount of time working on this elegant, elaborate post to a question, only to find out it has been deleted...well, that's perhaps a different story altogether =)