Subsequence - 392. Is Subsequence

1. Is Subsequence

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?

Credits: Special thanks to @pbrother for adding this problem and creating all test cases.

思路：

用贪心的思想，找出s串中第一个在t中出现的位置。

代码：

java：

class Solution {

    public boolean isSubsequence(String s, String t) {
        if (s == null || s.length() == 0) return true;
        if (t == null || t.length() == 0) return false;
        
       /* int index = 0;
        for (int i = 0; i < t.length(); i++) {
            if (s.charAt(index) == t.charAt(i)) index++;
            if (index == s.length()) return true;
        }
        
        return false;*/
        
        int prev = -1;
        for (int i = 0; i < s.length(); i++) {
            int index = t.indexOf(s.charAt(i), prev+1);
            if (index < 0) {
                return false;
            }
            prev = index;
        }
        return true;
    }
}