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.
当t中找到匹配的字符时，迭代t，s的先行指数。
只要它到达t的末尾，或者我们发现s中的所有字符，停止循环。
循环后，如果s的指数等于其长度，则返回true。 否则返回false。
bool isSubsequence(string s, string t) {
int sLength = s.length(), sIndex = 0, tLength = t.length();
for (int i=0; i<tLength && sIndex<sLength; i++)
if (t[i]==s[sIndex]) sIndex++;
return sIndex==sLength;
}