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"

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?

題目大意：判斷 字串s 是否由 字串t中的某些字元 ，按照原始的相對順序構成的。

解題思路：同時遍歷s和t，如果字元不相同，則右移t中的遍歷下標；如果字元相同，則同時右移s和t中的遍歷下標。如果t遍歷完，而s還沒遍歷完，則返回false，否則返回true。程式碼如下：（11ms，beats 83.94%）

public class Solution {
    public boolean isSubsequence(String s, String t) {
        char[] ss = s.toCharArray(), tt = t.toCharArray();
		int i, j = 0;
		for (i = 0; i < ss.length; i++) {
			while (j < tt.length && tt[j] != ss[i])
				j++;
			if (j == tt.length)
				return false;
			j++;
		}
		return true;
    }
}