Is Subsequence

Problem Description

Given two strings s and t, return true if s is a subsequence of t, or false otherwise.

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).

Examples

Example 1:

Input: s = "abc", t = "ahbgdc"
Output: true

Example 2:

Input: s = "axc", t = "ahbgdc"
Output: false
 

Complexity Analysis

*** Time Complexity:** $O(m+k*nlogm)$

*** Space Complexity:** $O(m)$

Constraints

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

Solution

Approach

To check if 's' is a subsequence of 't', use a two-pointer approach where one pointer traverses 's' and the other traverses 't'. If all characters of 's' are found in order within 't', return 'true'; otherwise, return 'false'. For handling many such checks efficiently, preprocess 't' into a mapping of character positions to quickly locate characters and verify subsequences, which speeds up the process for large numbers of queries.

Code in Different Languages

C++

Written by @ImmidiSivani

#include <string>

class Solution {
public:
    bool isSubsequence(std::string s, std::string t) {
        int i = 0; 
       int j = 0; 
    
        while (i < s.length() && j < t.length()) {
            if (s[i] == t[j]) {
                i++; 
            }
            j++; }

        return i == s.length();
    }
};

Java

Written by @ImmidiSivani

class Solution {
    public boolean isSubsequence(String s, String t) {
        int i = 0; 
        int j = 0; 
      
        while (i < s.length() && j < t.length()) {
            if (s.charAt(i) == t.charAt(j)) {
                i++; 
            }
            j++; 
        }

        return i == s.length();
    }
}

Python

Written by @ImmidiSivani

class Solution:
    def isSubsequence(self, s: str, t: str) -> bool:
        i=0
        j=0

        while i<len(s) and j<len(t):
            if s[i]==t[j]:
                i+=1
            j+=1
        return i==len(s)

References

• LeetCode Problem: Is Subsequence

• Solution Link: Is Subsequence