题目：

https://leetcode.com/problems/distinct-subsequences/?tab=Description

一般没有明显思路的情况下，都要想想DP，用下Divide-and-Conque

下面的思路很好，很清晰

https://discuss.leetcode.com/topic/9488/easy-to-understand-dp-in-java/2

一个矩阵，如果字符不一样，那就用左侧的；如果字符一样，那就左侧+上方的。

An example:
S: [acdabefbc] and T: [ab]

first we check with a:

           *  *
      S = [acdabefbc]
mem[1] = [0111222222]
then we check with ab:

               *  * ]
      S = [acdabefbc]
mem[1] = [0111222222]
mem[2] = [0000022244]

很好的DP思路，字符串比较次数