这题感觉比N-Queen I 还简单啊。。
Diagonals 的size 为 2*n 引人深思啊。
45 degree line is y = x + b
135 degree line is y = -x + b
in another word, 45 degree y - x is constant, and 135 degree y + x is constant.
Here b is shifted to [0~2n), so y-x and y+x can be trackable.
