題目連結：https://leetcode.com/problems/is-subsequence/

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?

思路：只要用兩個指標來判斷當前兩個字串中的字元是否相等即可．

程式碼如下：

class Solution {
public:
    bool isSubsequence(string s, string t) {
        if(s.size()>t.size()) return false;
        int len1 = s.size(), len2 = t.size(), k1 = 0, k2 = 0;
        while(k1 < len1 && k2 < len2)
        {
            if(s[k1]==t[k2]) k1++;
            k2++;
        }
        return k1 == len1;
    }
};