Is Subsequence

Problem

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).

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?

Constraints

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

Solution

The problem Is Subsequence can be solved using two pointers to iterate through t while finding letters from s.

Implementation

class Solution
{
  public:
    bool isSubsequence(string s, string t)
    {
        int i = 0;
        for (int j = 0; i < s.length() && j < t.length(); j++)
            if (s[i] == t[j])
                i += 1;

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