<p>You are given a string <code>s</code>, an integer <code>k</code>, a letter <code>letter</code>, and an integer <code>repetition</code>.</p> <p>Return <em>the <strong>lexicographically smallest</strong> subsequence of</em> <code>s</code><em> of length</em> <code>k</code> <em>that has the letter</em> <code>letter</code> <em>appear <strong>at least</strong></em> <code>repetition</code> <em>times</em>. The test cases are generated so that the <code>letter</code> appears in <code>s</code> <strong>at least</strong> <code>repetition</code> times.</p> <p>A <strong>subsequence</strong> is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters.</p> <p>A string <code>a</code> is <strong>lexicographically smaller</strong> than a string <code>b</code> if in the first position where <code>a</code> and <code>b</code> differ, string <code>a</code> has a letter that appears earlier in the alphabet than the corresponding letter in <code>b</code>.</p> <p> </p> <p><strong class="example">Example 1:</strong></p> <pre> <strong>Input:</strong> s = "leet", k = 3, letter = "e", repetition = 1 <strong>Output:</strong> "eet" <strong>Explanation:</strong> There are four subsequences of length 3 that have the letter 'e' appear at least 1 time: - "lee" (from "<strong><u>lee</u></strong>t") - "let" (from "<strong><u>le</u></strong>e<u><strong>t</strong></u>") - "let" (from "<u><strong>l</strong></u>e<u><strong>et</strong></u>") - "eet" (from "l<u><strong>eet</strong></u>") The lexicographically smallest subsequence among them is "eet". </pre> <p><strong class="example">Example 2:</strong></p> <img alt="example-2" src="https://assets.leetcode.com/uploads/2021/09/13/smallest-k-length-subsequence.png" style="width: 339px; height: 67px;" /> <pre> <strong>Input:</strong> s = "leetcode", k = 4, letter = "e", repetition = 2 <strong>Output:</strong> "ecde" <strong>Explanation:</strong> "ecde" is the lexicographically smallest subsequence of length 4 that has the letter "e" appear at least 2 times. </pre> <p><strong class="example">Example 3:</strong></p> <pre> <strong>Input:</strong> s = "bb", k = 2, letter = "b", repetition = 2 <strong>Output:</strong> "bb" <strong>Explanation:</strong> "bb" is the only subsequence of length 2 that has the letter "b" appear at least 2 times. </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>1 <= repetition <= k <= s.length <= 5 * 10<sup>4</sup></code></li> <li><code>s</code> consists of lowercase English letters.</li> <li><code>letter</code> is a lowercase English letter, and appears in <code>s</code> at least <code>repetition</code> times.</li> </ul>