Hello,

I was wondering if there's any trick that allows us to iterate over the boundary of an $$$N \times M$$$ grid using a single loop instead of 4 or 2.

To iterate over the whole grid for example we can iterate from 0 to $$$NM$$$ and have $$$i = x / M$$$ and $$$j = x \% M$$$.

So is there any smart thing like this to do for the boundary only (i.e pairs $$$(i, j)$$$ such that either $$$i = 0$$$ or $$$i = N - 1$$$ or $$$j = 0$$$ or $$$j = M - 1$$$)

Thanks for future help!