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 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?

分析

给定两个字符串，判断一个字符串s是不是另外一个字符串t的子串，这里子串的意思是t删除一部分字符可以得到s，没有连续的要求。
因为没有连续的要求，所以只需要判断t中是否有s中所有字符且排列顺序一致，那就依次遍历就可以了。

代码

public boolean isSubsequence(String s, String t) {
    if(s.length() == 0) return true;
    int i = 0, j = 0;
    while(i < s.length() && j < t.length()){
        if(s.charAt(i) == t.charAt(j)){
            i++;
        }
        j++;
    }
    return i == s.length() ? true : false;
}