392. Is Subsequence_qt issubsequence-优快云博客

本文介绍了一种高效的方法来判断一个字符串是否为另一个字符串的子序列。通过一次遍历，该方法能够在大型目标字符串中快速定位到短字符串的所有字符，适用于处理大量查询的情况。

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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?

要保持原顺序不变才算子序列，那么只要扫描一遍s，对于字符s.CharAt(i)，在t中的位置如果是j，则下次查找只从串t的j+1位置开始。

若s中的字符在t中检索到了末尾仍然没有找到则失败（s不是t的子序列）

public class Solution {
    public boolean isSubsequence(String s, String t)
	{
		int slen=s.length(),tlen=t.length();
		int start=0;
		for(int i=0;i<slen;i++)
		{
			char c=s.charAt(i);
			int j;
			for(j=start;j<tlen;j++)
				if(t.charAt(j)==c)
				{
					start=j+1;
					break;
				}
			if(j==tlen)
				return false;
		}
		return true;
	}
}