Distinct Subsequences

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).
S = "rabbbit", T = "rabbit"
Return 3.

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