How do I find all files and directories writable by a specific user?

Under Linux, how can I find all the files and directories that are writable (or, actually, not writable) by a particular user?

Edit: To clarify, I meant under a particular subdir, not systemwide. And yes, that means all the permutations and combinations of user, group and world writability that would allow that user to write. I know what the question entails semantically, I was hoping for a one- or few-liner to execute to get a list of these files.