涉及到对角线，反对角线时候，index应该如何取
假设matrix[n][m]
0 1 2 3
0 0 1 1 0
1 0 1 1 0
2 0 0 0 1
diag: (0, 1) (1 ,2) (2, 3)
可以这么想：从(0, 3)出发，到各个点的距离相同
adiag: (0, 2) (1, 1), (2, 0)
可以这么想：从(0, 0)出发，到各个点的距离相同
int[] diag = new int[n + m - 1]
int[] adiag = new int[n + m - 1]
for i
for j
diag[m - 1 - j + i] = ...
adiag[i + j]
X. . .
X . . X
X . . X
. . X .
. . X .
[2, 0], [2, 3], [4, 2]
if (board[i][j] == 'X' && (i == n - 1 || board[i + 1][j] == '.') && (j == m - 1 || board[i][j + 1] == '.')) {
ans += 1;
}
