Description

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 sis 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?

Credits:
Special thanks to @pbrother for adding this problem and creating all test cases.

Solution

Two-pointer, time O(m + n), space O(1)

起初用DP做过了，后来发现并不需要，双指针法就可以解决了。

class Solution {
    public boolean isSubsequence(String s, String t) {
        int i = 0;
        
        for (int j = 0; j < t.length() && i < s.length(); ++j) {
            if (s.charAt(i) == t.charAt(j)) {
                ++i;
            }
        }
        
        return i == s.length();
    }
}