392. Is Subsequence

1. Description

Given a string s and a string t, check if s is subsequence of t.
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).

2. Example

Example 1:
Input: s = “abc”, t = “ahbgdc”
Output: true

Example 2:
Input: s = “axc”, t = “ahbgdc”
Output: false

3. Constraints

• 0 <= s.length <= 100
• 0 <= t.length <= $10^4$
• Both strings consists only of lowercase characters.

4. Solutions

Two Pointers

n = target.size()
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    bool isSubsequence(const string &sub, const string &target) {
        int sub_index = 0;
        for (int i = 0; i < target.size() && sub_index < sub.size(); ++i) {
            if (sub[sub_index] == target[i]) {
                ++sub_index;
            }
        }

        return sub_index == sub.size();
    }
};