LeetCode: Distinct Subsequences（不同子序列的个数）

题目描述：

Given a string S and a string T, count the number of distinct subsequences ofT inS.

A subsequence of a string is a new string which is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (ie,"ACE" is a subsequence of"ABCDE" while "AEC" is not).

Here is an example:
S = "rabbbit", T = "rabbit"

Return 3.

简单翻译一下，给定两个字符串S和T，求S有多少个不同的子串与T相同。S的子串定义为在S中任意去掉0个或者多个字符形成的串。

递归求解：

首先找到在S中与T的第一个字符相同的字符，从这个字符开始，递归地求S和T剩下的串。T为空串时，返回1。因为空串本身是另外一个串的一个子序列。这个算法实现简单，但是果然不出意料，大集合超时。

Java代码：

  public int numDistinct(String S, String T) {
    // Start typing your Java solution below
    // DO NOT write m