枚举2树下时，移动次数要从1开始！！！！！

for (int j = 1  <---------   ; j <= w && j <= i; j++) f[i][j][1] = max(f[i - 1][j][1], f[i - 1][j - 1][0]) + (a[i] == 2 ? 1 : 0);
for (int j = 0  <---------   ; j <= w && j <= i; j++) f[i][j][0] = max(f[i - 1][j][0], f[i - 1][j - 1][1]) + (a[i] == 1 ? 1 : 0);