distinctSubsequence

Problem

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

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.

Solution

The topic sentence is poorly written. It would be better if you understand it as: 'count the numbrer of S' subsequences that is equal to T'

That said, this is a regular DP problem