I think this problem is not very hard and it's interesting.
Description: ~~~~~ You are given $$$2$$$ arrays $$$a$$$ and $$$b$$$ and the length of them is $$$n$$$.
$$$1$$$, $$$3$$$, $$$5$$$, ......, $$$2 * n - 1$$$ is in $$$a$$$.
2, 4, 6, ......, 2 * n is in b.
~~~~~