392. Is Subsequence

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;
}
};