Distinct Subsequences

Given two strings s and t, return the number of distinct subsequences of s which equals t.

A subsequence of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.

For example, "ace" is a subsequence of "abcde" while "aec" is not.

The test cases are generated so that the answer fits on a 32-bit signed integer.