Is Subsequence

Given a string s and a string t, check if s is subsequence of t.

You may assume that there is only lower case English letters in both s and t. t is potentially a very long (length ~= 500,000) string, and s is a short string (<=100).

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

Example 1:
s = "abc", t = "ahbgdc"

Return true.

Example 2:
s = "axc", t = "ahbgdc"

Return false.

Follow up:
If there are lots of incoming S, say S1, S2, ... , Sk where k >= 1B, and you want to check one by one to see if T has its subsequence. In this scenario, how would you change your code?

Go through the string and whenever pass all chars that don't match. In the end a subsequence of the string should reach to the end.

public boolean isSubsequence(String s, String t) {
        int lenS = s.length(), lenT = t.length();
        if (lenS == 0) {
            return true;
        }
        if (lenS > lenT) {
            return false;
        }
        
        int posS = 0, posT = 0;
        while (posS < lenS && posT < lenT) {
            while (posT < t.length() && t.charAt(posT) != s.charAt(posS)) {
                posT++;
            }
            posS++;
            posT++;
        }
        return posS == lenS;
    }