Raf’s Problem

Raf – a colleague – gave me the following problem:

Given a String (x) containing only characters a-z, write a function (f) that returns a base 10 integer, which converts the String as if it were a base 26 numeral. Function f is bijective.

Here are some example runs:

x      | f(x)
empty  | 0
a      | 1
b      | 2
z      | 26
aa     | 27
az     | 52
ba     | 53
bz     | 78
aaa    | 703
aaz    | 728
aza    | 1353

Using your preferred programming language, implement function f.

My solution is in a comment on this page. What is yours?