Given two strings `s` and `t`, return `true` if `s` is a subsequence of `t`, or `false` otherwise.

A subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., `"ace"` is a subsequence of `"abcde"` while `"aec"` is not).

Example 1:

    Input: s = "abc", t = "ahbgdc"
    Output: true

Example 2:

    Input: s = "axc", t = "ahbgdc"
    Output: false

Constraints:

- `0 <= s.length <= 100`
- `0 <= t.length <= 104`
- `s` and `t` consist only of lowercase English letters.

Follow up: Suppose there are lots of incoming `s`, say `s1, s2, ..., sk` where `k >= 109`, and you want to check one by one to see if `t` has its subsequence. In this scenario, how would you change your code?