Q: Suppose a language A is NP-complete. Is this following statement correct?:
A: If there is a polynomial algorithm to solve A, then it can be used to all the problems in NP.
My (and some others) first thoughts were that this answer is correct. But if you look at the following picture you can see that there exist problems which are only! NP. So let's take such a problem which is not NP-complete but NP. Thus a polynomial algorithm which can be used to solve any problem which is NP-complete can not be used to solve this problem which is only! NP. Is this correct?