392. Is Subsequence（补做上周忘记的那份）

Description:

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?

Credits:
Special thanks to @pbrother for adding this problem and creating all test cases.

简要题解：

采用双重循环（运行效率还是不错的，击败80+%的提交）。以s为外层循环，t为内层循环。顺序寻找s的字符是否在t内。一旦不在，便立刻return false。在的话，用一个pos标记在t中找到匹配字符的位置的下一个位置。下一次循环，t从pos位置开始。两层循环都顺利结束。就表明s是t的一个子串

代码：

class Solution {public:    bool isSubsequence(string s, string t) {        int pos = 0;        bool flag;        for (int i = 0; i < s.size(); i++) {            flag = false;            for (int j = pos; j < t.size(); j++)                if (s[i] == t[j]) {                    flag = true;                    pos = j + 1;                    break;                }                        if (!flag)                return false;        }        return true;    }};