Topic: https://nanti.jisuanke.com/t/41414
The meaning of problems: seek length n of the k-th bell number, i is the first bit in the selected range of the maximum value +1 1- (i-1) bits, the first bit is fixed to A, by lexicographic order
Ideas: establishment dp [n] [i] [j] representative length n, i-th layer, the maximum value of the first bit is 1-i j is the number of programs