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

A string's subsequence is a new string formed from the original string by deleting some (can be none) of the characters without disturbing the remaining characters' relative positions. (i.e., `"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.

Example 1:

    Input: s = "rabbbit", t = "rabbit"
    Output: 3
    Explanation:
    As shown below, there are 3 ways you can generate "rabbit" from S.
    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.
    babgbag
    babgbag
    babgbag
    babgbag
    babgbag

Constraints:

- `1 <= s.length, t.length <= 1000`
- `s` and `t` consist of English letters.