![\"example-2\"](\"https://assets.leetcode.com/uploads/2021/09/13/smallest-k-length-subsequence.png\")
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.
\n\nA 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.
\n
Example 1:
\n\n\nInput: s = "leet", k = 3, letter = "e", repetition = 1\nOutput: "eet"\nExplanation: There are four subsequences of length 3 that have the letter 'e' appear at least 1 time:\n- "lee" (from "leet")\n- "let" (from "leet")\n- "let" (from "leet")\n- "eet" (from "leet")\nThe lexicographically smallest subsequence among them is "eet".\n\n\n
Example 2:
\n\n\nInput: s = "leetcode", k = 4, letter = "e", repetition = 2\nOutput: "ecde"\nExplanation: "ecde" is the lexicographically smallest subsequence of length 4 that has the letter "e" appear at least 2 times.\n\n\n
Example 3:
\n\n\nInput: s = "bb", k = 2, letter = "b", repetition = 2\nOutput: "bb"\nExplanation: "bb" is the only subsequence of length 2 that has the letter "b" appear at least 2 times.\n\n\n
\n
Constraints:
\n\n1 <= repetition <= k <= s.length <= 5 * 104s consists of lowercase English letters.letter is a lowercase English letter, and appears in s at least repetition times.给你一个字符串 s ，一个整数 k ，一个字母 letter 以及另一个整数 repetition 。
返回 s 中长度为 k 且 字典序最小 的子序列，该子序列同时应满足字母 letter 出现 至少 repetition 次。生成的测试用例满足 letter 在 s 中出现 至少 repetition 次。
子序列 是由原字符串删除一些（或不删除）字符且不改变剩余字符顺序得到的剩余字符串。
\n\n字符串 a 字典序比字符串 b 小的定义为：在 a 和 b 出现不同字符的第一个位置上，字符串 a 的字符在字母表中的顺序早于字符串 b 的字符。
\n\n
示例 1：
\n\n\n输入：s = \"leet\", k = 3, letter = \"e\", repetition = 1\n输出：\"eet\"\n解释：存在 4 个长度为 3 ，且满足字母 'e' 出现至少 1 次的子序列：\n- \"lee\"（\"leet\"）\n- \"let\"（\"leet\"）\n- \"let\"（\"leet\"）\n- \"eet\"（\"leet\"）\n其中字典序最小的子序列是 \"eet\" 。\n\n\n
示例 2：
\n\n
\n输入：s = \"leetcode\", k = 4, letter = \"e\", repetition = 2\n输出：\"ecde\"\n解释：\"ecde\" 是长度为 4 且满足字母 \"e\" 出现至少 2 次的字典序最小的子序列。\n\n\n
示例 3：
\n\n\n输入：s = \"bb\", k = 2, letter = \"b\", repetition = 2\n输出：\"bb\"\n解释：\"bb\" 是唯一一个长度为 2 且满足字母 \"b\" 出现至少 2 次的子序列。\n\n\n
\n\n
提示：
\n\n1 <= repetition <= k <= s.length <= 5 * 104s 由小写英文字母组成letter 是一个小写英文字母，在 s 中至少出现 repetition 次\\u7248\\u672c\\uff1a \\u7f16\\u8bd1\\u65f6\\uff0c\\u5c06\\u4f1a\\u91c7\\u7528 \\u4e3a\\u4e86\\u4f7f\\u7528\\u65b9\\u4fbf\\uff0c\\u5927\\u90e8\\u5206\\u6807\\u51c6\\u5e93\\u7684\\u5934\\u6587\\u4ef6\\u5df2\\u7ecf\\u88ab\\u81ea\\u52a8\\u5bfc\\u5165\\u3002<\\/p>\"],\"java\":[\"Java\",\" \\u7248\\u672c\\uff1a \\u4e3a\\u4e86\\u65b9\\u4fbf\\u8d77\\u89c1\\uff0c\\u5927\\u90e8\\u5206\\u6807\\u51c6\\u5e93\\u7684\\u5934\\u6587\\u4ef6\\u5df2\\u88ab\\u5bfc\\u5165\\u3002<\\/p>\\r\\n\\r\\n \\u5305\\u542b Pair \\u7c7b: https:\\/\\/docs.oracle.com\\/javase\\/8\\/javafx\\/api\\/javafx\\/util\\/Pair.html <\\/p>\"],\"python\":[\"Python\",\" \\u7248\\u672c\\uff1a \\u4e3a\\u4e86\\u65b9\\u4fbf\\u8d77\\u89c1\\uff0c\\u5927\\u90e8\\u5206\\u5e38\\u7528\\u5e93\\u5df2\\u7ecf\\u88ab\\u81ea\\u52a8 \\u5bfc\\u5165\\uff0c\\u5982\\uff1aarray<\\/a>, bisect<\\/a>, 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