I solved the second one in a couple minutes in my head with value substitutions. Unless you were to do with with a pen and paper, it'd be pretty difficult to pick up on the closed loops.

Generating the following list took less than 30 secs when I looked it over, so obviously the example of it taking a long time to solve is a bit contrived. What threw me off a bit at first was that the number 4 was not used at all. I spent a little time trying to figure out why that was excluded before moving on.

0 = 1 1 = 0 2 = 0 3 = 0 5 = 0 6 = 1 7 = 0 8 = 2 9 = 1