.gitignore: add data to ease test precess
121.c: done
Mind for the even degree. If all islands are of even degree, we
cannot pick a random one as stated by others.
We should firstly pick those whose degree are more than 2.

test case: (wrong answer by most of the code)
7
2 3 0
1 3 0
1 2 4 5 0
3 5 0
3 4 0
3 7 0
3 6 0