## [392\. Is Subsequence](https://leetcode.com/problems/is-subsequence/)

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?

:::success
- 時間複雜度：$O()$
- 空間複雜度：$O()$
:::