原题链接在这里：https://leetcode.com/problems/is-subsequence/description/
题目：
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 sis 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?
题解：
Two pointers. 当所指char相同就移动s的index, 看s的index能不能移动到末尾.
Time Complexity: O(s.length()+t.length()).
Space: O(1).
AC Java:
1 class Solution { 2 public boolean isSubsequence(String s, String t) { 3 if(s == null || s.length() == 0){ 4 return true; 5 } 6 7 if(t == null || t.length() == 0){ 8 return false; 9 } 10 11 int i = 0; 12 int j = 0; 13 while(j < t.length()){ 14 if(s.charAt(i) == t.charAt(j)){ 15 i++; 16 if(i == s.length()){ 17 return true; 18 } 19 } 20 j++; 21 } 22 return false; 23 } 24 }