115. Distinct Subsequences

Given a stringSand a stringT, count the number of distinct subsequences ofSwhich equalsT.

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"

Return3.

思路： 太难，不会考，感兴趣看这

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

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