115. Distinct Subsequences
https://leetcode.com/problems/distinct-subsequences/
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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. (ie, "ACE"
is a subsequence of "ABCDE"
while "AEC"
is not).
It's guaranteed the answer fits on a 32-bit signed integer.
Example 1:
Input: S ="rabbbit"
, T ="rabbit" Output: 3
Explanation: As shown below, there are 3 ways you can generate "rabbit" from S. (The caret symbol ^ means the chosen letters)rabbbit
^^^^ ^^rabbbit
^^ ^^^^rabbbit
^^^ ^^^
Example 2:
Input: S ="babgbag"
, T ="bag" Output: 5
Explanation: As shown below, there are 5 ways you can generate "bag" from S. (The caret symbol ^ means the chosen letters)babgbag
^^ ^babgbag
^^ ^babgbag
^ ^^babgbag
^ ^^babgbag
^^^