Do you know what a linked list is? Do you know what a binary search is? I'm confident that if you research these two terms, you'll be able to figure this one out.
I will give you a hint, in the form of a rhetorical question: how do you access element n in a linked list?
Partially correct.
A linked list is basically a wrapper for a chain of nodes. Each node points to the next element and (I'm not sure if this is how it works in the STL) possibly to the one before it. To move through the list, the wrapper must examine each pointer to move down the elements until the desired element is found.
Anyway try googling your question first. That works in a lot of cases.