392. Is Subsequence （子序列判断）

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?

题目大意：判断 字符串s 是否由 字符串t中的某些字符 ，按照原始的相对顺序构成的。

解题思路：同时遍历s和t，如果字符不相同，则右移t中的遍历下标；如果字符相同，则同时右移s和t中的遍历下标。如果t遍历完，而s还没遍历完，则返回false，否则返回true。代码如下：（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;    }}