Question

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?

Solution

class Solution {
    public boolean isSubsequence(String s, String t) {
        if(t.length() < s.length()) return false;

        int tIdx = 0;
        for(char c: s.toCharArray()) {
            if(tIdx == t.length())
                return false;
            while(t.charAt(tIdx++) != c)
                if(tIdx == t.length())
                    return false;
        }
        return true;
    }
}