# Leetcode-Is Subsequence Date: 2021-05-18 #### **问题** Given two strings `s` and `t`, check if `s` is a **subsequence** of `t`. A **subsequence** of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., `"ace"` is a subsequence of `"abcde"` while `"aec"` is not).   **Example 1:** **Input:** ``` s = "abc", t = "ahbgdc" ``` **Output:** ``` true ``` **Example 2:** **Input:** ``` s = "axc", t = "ahbgdc" ``` **Output:** ``` false ```   **Constraints:** - `0 <= s.length <= 100` - `0 <= t.length <= 10`4 - `s` and `t` consist only of lowercase English letters.   **Follow up:** If there are lots of incoming `s`, say `s`1`, s`2`, ..., s`k where `k >= 10`9, and you want to check one by one to see if `t` has its subsequence. In this scenario, how would you change your code? #### **解答：** 从左到右逐个比较主字符串和子字符串 - 相等时，同时移动主串和子字符串索引指向下一个字符 - 不相等，仅移动主串索引指向下一个字符 如果发现子数组的索引已经移动到了数组的末尾，即等于字串长度，表明找到字符串。 ``` class Solution: def isSubsequence(self, s: str, t: str) -> bool: sIdx = 0 tIdx = 0 while sIdx < len(s) and tIdx