[LeetCode] Problem 392 - Is Subsequence

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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

No.1

s = “abc”, t = “ahbgdc”

Return true.

No.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?

Code

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public boolean isSubsequence(String s, String t) {
    int idx = 0;
    int n = t.length();

    for (char ch : s.toCharArray()) {
        while (idx < n && ch != t.charAt(idx))
            idx++;
        
        if (idx++ == n)
            return false;
    }

    return true;
}