A - CF1368B Codeforces Subsequences
https://www.luogu.com.cn/article/csqjsb40
B - Good Subarrays
令 \(b_i = a_i - 1\)，则条件等价于 \(\sum_{i=l}^r b_i = 0\)。计算前缀和 \(S_k = \sum_{i=1}^k b_i\)，则子数组 \([l,r]\) 和为 \(S_r - S_{l-1} = 0\) 即 \(S_r = S_{l-1}\)。
因此用哈希表记录每个前缀和出现的次数，遍历时累加当前前缀和之前出现的次数，特别地当 \(S_r = 0\) 时额外计入 \([1,r]\)。
D - Task On The Board
https://www.luogu.com.cn/article/4oihz0l3
E - Necklace Assembly
https://www.luogu.com.cn/article/d93oy645
F - Graph Coloring
见：https://www.luogu.com.cn/article/4d5ul5ts