问题描述：字符序列的子序列是指从给定字符序列中随意地（不一定连续）去掉若干个字符（可能一个也不去掉）后所形成的字符序列。令给定的字符序列X=“x0，x1，…，xm-1”，序列Y=“y0，y1，…，yk-1”是X的子序列，存在X的一个严格递增下标序列<i0，i1，…，ik-1>，使得对所有的j=0，1，…，k-1，有xij=yj。例如，X=“ABCBDAB”，Y=“BCDB”是X的一个子序列。 -Problem Descr iption: character sequence is the sequence of a given sequence of characters from arbitrarily (not necessarily consecutive) to remove a number of characters (perhaps a not removed) after the formation of character sequence. So that a given character sequence X = " x0, x1, ..., xm-1" , the sequence Y = " y0, y1, ..., yk-1"　is a sequence of X, X, there is a strictly increasing sequence subscr ipt <i0，i1，…，ik-1> So for all j = 0,1, ..., k-1, with xij = yj. For example, X = " ABCBDAB" , Y = " BCDB"　is a sequence of X.