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.

# 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.

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.