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.

 1 // check if s is subsequence of t. 2     public static boolean isSubsequence(String s, String t) { 3         if (t.length() < s.length()) 4             return false; 5         //　查找的启示位置 6         int prev = 0; 7         // 遍历s的字符 8         for (int i = 0; i < s.length(); i++) { 9             // 依次获取每个字符10             char tempChar = s.charAt(i);11             // 在上一次查找出字符的位置之后，查找t是否包含指定字符12             prev = t.indexOf(tempChar, prev);13             // t中不包含s的字符14             if (prev == -1)15                 return false;16             // 下一次查找位置为：本次查找出字符的下一个位置17             prev++;18         }19         return true;20     }

1. 题目考察如何判断s是t的字串
2. 用到的API：
   1. String.charAt(int i)：Returns the <code>char</code> value at the specified index.
   2. String.indexOf(char , index)：Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.