# LeetCode 392. Is Subsequence

## Description

https://leetcode.com/problems/is-subsequence/

Given two strings `s` and `t`, check if `s` is a subsequence of `t`.

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: If 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?

## Explanation

Check if iterating the t string can find all s string in sequence.

## Python Solution

``````class Solution:
def isSubsequence(self, s: str, t: str) -> bool:
if not s:
return True

i = 0

for c in t:
if c == s[i]:
i += 1
if i >= len(s):
break

if i == len(s):
return True

return False``````
• Time Complexity: O(N).
• Space Complexity: O(1).