2030 - Smallest K-Length Subsequence With Occurrences of a Letter.

Hard

You are given a string s, an integer k, a letter letter, and an integer repetition.

Return the lexicographically smallest subsequence of s of length k that has the letter letter appear at least repetition times. The test cases are generated so that the letter appears in s at least repetition times.

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.

A string a is lexicographically smaller than a string b if in the first position where a and b differ, string a has a letter that appears earlier in the alphabet than the corresponding letter in b.

Example 1:

Input: s = "leet", k = 3, letter = "e", repetition = 1

Output: "eet"

Explanation: There are four subsequences of length 3 that have the letter 'e' appear at least 1 time:

• "lee" (from "leet")

• "let" (from "leet")

• "let" (from "leet")

• "eet" (from "leet")

The lexicographically smallest subsequence among them is "eet".

Example 2:

Input: s = "leetcode", k = 4, letter = "e", repetition = 2

Output: "ecde"

Explanation: "ecde" is the lexicographically smallest subsequence of length 4 that has the letter "e" appear at least 2 times.

Example 3:

Input: s = "bb", k = 2, letter = "b", repetition = 2

Output: "bb"

Explanation: "bb" is the only subsequence of length 2 that has the letter "b" appear at least 2 times.

Constraints:

• <code>1 <= repetition <= k <= s.length <= 5 * 10⁴</code>

• s consists of lowercase English letters.

• letter is a lowercase English letter, and appears in s at least repetition times.