fill the black board with two color

This problem is kind like 8 queen problem.

analysis process:

sub problem: if you paint the first one, you will decide how to paint
the second one, this will be a sub problem.

basecase:

you paint all the board, so you will done.

if you can paint the second, so you can recursively paint the next one.

limitation:

n>=2 && c[n-1] == c[n] && c[n] == c[n-2]