I know this is a super-unpopular opinion these days, but...
Why can't you program the simple problems that interviewers use for whiteboard coding without Stack Overflow?
It's not even like interviewers are looking for syntax-perfect code (which you wouldn't need Stack Overflow for anyway, the compiler is what catches that for you). I've never had an interviewer that would quibble over a missed semi-colon here or there. Hell, most of the time I end up using "begin/end" style pseudo-code instead of curly-braces, because curly-braces are stupidly hard to draw well on a whiteboard.
And even if you don't know off the top of your head what "invert binary tree" means, every interviewer I've ever had has been more than willing to answer any questions I have about the problem. Ask questions about requirements and you should be able to figure it out. If, once you know what the inputs and outputs of "invert binary tree" should be and you still can't do it, I seriously gotta question what the hell you're doing interviewing for the job of programmer. I once passed an interview for a job I wasn't even applying for, even though I didn't know off the top of my head what "LRU cache" meant, by asking enough questions ("oh, LRU means least-recently used? Well then, something like this?")
I tend to think of whiteboard coding problems as being ineffective for interviewing because the problems are so simple. To me, anybody should be able to solve these problems. I know a lot of people can't, and this serves as a quick filter to winnow them out. But they are so ludicrously easy I just don't get how you can say you need Stack Overflow to solve these problems.
I'm with Jonathan Blow on this one: https://twitter.com/mxcl/status/608682016205344768