115. Distinct Subsequences

Given a string S and a string T, count the number of distinct subsequences of S which equals T.

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. (i.e., “ACE” is a subsequence of “ABCDE” while “AEC” is not).

Here is an example:

S = “rabbbit”, T = “rabbit”

Return 3.

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历史题目和总结见公众号「每日一道算法题」

https://leetcode.com/problems/distinct-subsequences/description/