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?

思路：

两个指针分别指向s和t的头部，然后比较两个字符是否相等，如果相等同时后移，不相等就将t的指针向后移一位。

bool isSubsequence(string s, string t)
     {
         int  lens = s.length(), lent = t.length(), indexS = 0, indexT = 0;
         if (lens == 0)return true;
         while (indexT < lent)
         {
             if (t[indexT]==s[indexS])
             {
                 indexS++;
                 if (indexS == lens)return true;
             }
             indexT++;
         }
         return false;
     }

参考：

https://discuss.leetcode.com/topic/57147/straight-forward-java-simple-solution

[leetcode-392-Is Subsequence]