Problem A subsequence of a given sequence is just the given sequence with some elements (possibly none) left out. Formally, given a sequence X = x1x2 . . . xm, another sequence Z = z1z2 . . . zk is a subsequence of X if there exists a strictly increasing sequence of indices of X such that for all j = 1, 2, . . . , k, we have xij = zj . For example, Z = bcdb is a subsequenc..