Choose one of the stack implementations and implement a method with this specification:

Object itemAt(int n)
// Precondition: 0 <= n and n < size( ).
// Postcondition: The return value is the
// item that is n from the top (with the top at
// n = 0, the next at n = 1, and so on). The
// stack is not changed.

Throw a NoSuchElementException if the precondition is violated (from java.util.NoSuchElementException).