## Solution

Better to be classified as implementation problem. Lots of details. Get Lots of WA.

Attention: You are not allowed to use leading zeros for $x$ and $y$.

So b--, because the first one must be 1.

Want have $k$ ones. So it's very clear that we can have this pair of $i$ and $j$. So that $i<j$ and $x_i=1,\ y_i=0, x_j=0, y_j=1$ and for any $p\in{(i,j)}$ have $x_p=y_p$. Then we just need adjust the position of $i$, $j$. Or return no.

## Code

First two WA5 is cause by my misunderstanding of this problem. After that, just changed the code base on WA5's code. But the code structure still not changed.

Then I change the way to current code. It's much better. But still lots of WA.