2486. Append Characters to String to Make Subsequence

Problem

You are given two strings s and t consisting of only lowercase English letters.

Return **the minimum number of characters that need to be appended to the end of *s* so that t becomes a subsequence of **s.

A subsequence is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters.

  Example 1:

Input: s = "coaching", t = "coding"
Output: 4
Explanation: Append the characters "ding" to the end of s so that s = "coachingding".
Now, t is a subsequence of s ("coachingding").
It can be shown that appending any 3 characters to the end of s will never make t a subsequence.

Example 2:

Input: s = "abcde", t = "a"
Output: 0
Explanation: t is already a subsequence of s ("abcde").

Example 3:

Input: s = "z", t = "abcde"
Output: 5
Explanation: Append the characters "abcde" to the end of s so that s = "zabcde".
Now, t is a subsequence of s ("zabcde").
It can be shown that appending any 4 characters to the end of s will never make t a subsequence.

  Constraints:

• 1 <= s.length, t.length <= 105

• s and t consist only of lowercase English letters.

Solution (Java)

class Solution {
    public int appendCharacters(String s, String t) {
        int idx = 0;
        for (int i = 0; i < s.length(); ++i) {
            if (idx < t.length() && s.charAt(i) == t.charAt(idx)) {
                ++idx;
            }
        }
        return t.length() - idx;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).