392. Is Subsequence 待改进

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?

这个是在一个大字符串里找存不存在小字符串的序列，只要小字符串里的字母按顺序在大字符串里出现了就行，不一定要挨着。
我使用的方式是双指针，但是这个并没有对题目中提到的大数据量的情况做出优化：

var isSubsequence = function(s, t) {
    var i=0;
    var j=0;
    var nums = s.length;
    var numt = t.length;
    while(i<nums&&j<numt) {
        if (s[i]===t[j])
            i++;
        j++;
    }
    if (i===nums)
        return true;
    return false;
};