一個指標，從左到右遍歷

class Solution {
    public boolean isSubsequence(String s, String t) {
        if(s==null||t==null) return false;
        int i=0,j=0;
        int count=0;
        while(i<s.length()&&j<t.length()){
            if(s.charAt(i)==t.charAt(j)){
                i++;
                j++;
                count++;
            }
            else{
                j++;
            }
        }
        if(count==s.length()) return true;
        else return false;
    }
}