# [LeetCode] Top Interview 150 Problems Solving # 392 Is Subsequence

# **Understanding the Problem**

Given strings `s` and `t`, it is to tell whether `s` is the subsequence to `t`. It is not finding if the characters in `s` exist in `t`. Well basically it is, but the sequence matters too.

```python
# example
Input: s = "abc", t = "ahbgdc"
Output: true
```

# **Approach**

This would require a `for` loop to check each character. The loop will go through `t` as it may or may not contain `s`.

# **Solution**

I have declared `idx` and initialized it with 0, to start from the first index of `s`. If only `c` from the loop of `t` is identical to `s[idx]`, add up one to `idx` to move to the next sequence of `s`.

But it needs checking if idx is at the end of `s`. If it is at the end of `s`, there is no further need for checking the characters anymore, thus just return `True`.

```python
class Solution:
    def isSubsequence(self, s: str, t: str) -> bool:
        idx = 0
        for c in t:
            if idx >= len(s):
                return True
            elif c == s[idx]:
                idx += 1
        
        if idx == len(s):
            return True
        return False
```

# **Reflection**

The fact that I was trying to solve this with `sort()` or `set()` was a bit hilarious, but it was worth trying to see many test cases that went wrong with my code. It had 0ms time complexity which I am satisfied with.