392. Is Subsequence

Question

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).

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 <= 104
s and t consist only of lowercase English letters.

Follow up: Suppose 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?

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Approach

1. Iterate through t, and match against s start from the first index in s.
2. If there is a match, move to the next index in s, and also take note of the index of the previous matched element.
3. After iterate through all elements in t if index of s is not at the end, that means some elements weren't matched.

Solution

class Solution {
public:
    bool isSubsequence(string s, string t) {
        int j = 0;
        
        for(int i = 0; i < t.size(); i++) if(t[i] == s[j]) j++;
        
        if(j != s.size()) return false;
        
        return true;
    }
};