LeetCode 392. Is Subsequence



Given two strings s and t, check if s is a subsequence of t.

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


  • 0 <= s.length <= 100
  • 0 <= t.length <= 104
  • s and t consist only of lowercase English letters.

Follow up: If there are lots of incoming s, say s1, s2, ..., sk where k >= 109, and you want to check one by one to see if t has its subsequence. In this scenario, how would you change your code?


Check if iterating the t string can find all s string in sequence.

Python Solution

class Solution:
    def isSubsequence(self, s: str, t: str) -> bool:
        if not s:
            return True
        i = 0
        for c in t:
            if c == s[i]:
                i += 1
            if i >= len(s):
        if i == len(s):     
            return True
        return False
  • Time Complexity: O(N).
  • Space Complexity: O(1).

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