Calculating function time

Question

For an upcoming final I know theres going to be questions about how to calculate how long a function takes. I was never really able to understand it, and the prof is unavailable until the exam which is obviously too late. Does anyone know how to explain it? or at least very clear websites/tutorials? All of my google searches came up empty.
For instance, here is a question that has been given and I have no idea how to calculate it at all.
To within a constant factor, how much time does the following
Java method take, as a function of the length n of string s?
(reverse(s) produces the reversal of s. For example, reverse("abc")
= "cba".)
public static String reverse(String s)
int n = s.length();
int k = 0;
String result = "";
while(k < n)
result = s.charAt(k).toString() + result;
k = k + 1;
return result;
}Also, if you are able to explain how to write more efficient functions, that would be greatly appreciated as well.
Thanks

Answer 1

That's not phrased well. When he says "how much time", what he really means (if he's not insane) is how many steps are taken to run through the algorithm. This more-or-less maps to time spent running the algorithm, because the steps all take roughly the same amount of time to do.
This is often also called "complexity".
It's usually described in terms of "big-O notation", such as "O(nlogn)". O(nlogn) means that for an input data set of size n, the algorithm performs on the order of n * log(n) steps, times some constant.
I'm glossing over the details here myself. Look up "big-O notation" or "algorithm analysis" on wikipedia.