after you practice a couple hundred of them and learn how to approach it, you'll recognize which algorithms and data structures apply to a question almost immediately. from there, it's just quickly working out the edge cases and coding it up. it's a skill. it takes dozens (sometimes hundreds) of hours of practice to get good, but you can. when you're first starting out, it's a "how do I figure this out" problem. after a while, it's a "oh, this is like that other thing" recall practice. It's the SAT/ACT/GRE/LSAT/MCAT of software engineering. It's just a test that shows how good you are at the test. The important part is that you can demonstrate that you a) know the fundamentals of data structures and algorithms and 2) can cleanly break down a problem so that it's *easy* for you to code up a solution in 10-15 minutes. If it's hard to write the code, you need to work on your problem solving.