LeetCode-in-Java

2014. Longest Subsequence Repeated k Times

Hard

You are given a string s of length n, and an integer k. You are tasked to find the longest subsequence repeated k times in string 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.

A subsequence seq is repeated k times in the string s if seq * k is a subsequence of s, where seq * k represents a string constructed by concatenating seq k times.

• For example, "bba" is repeated 2 times in the string "bababcba", because the string "bbabba", constructed by concatenating "bba" 2 times, is a subsequence of the string "**b**a**bab**c**ba**".

Return the longest subsequence repeated k times in string s. If multiple such subsequences are found, return the lexicographically largest one. If there is no such subsequence, return an empty string.

Example 1:

Input: s = “letsleetcode”, k = 2

Output: “let”

Explanation: There are two longest subsequences repeated 2 times: “let” and “ete”. “let” is the lexicographically largest one.

Example 2:

Input: s = “bb”, k = 2

Output: “b”

Explanation: The longest subsequence repeated 2 times is “b”.

Example 3:

Input: s = “ab”, k = 2

Output: “”

Explanation: There is no subsequence repeated 2 times. Empty string is returned.

Constraints:

• n == s.length
• 2 <= n, k <= 2000
• 2 <= n < k * 8
• s consists of lowercase English letters.

Solution

import java.util.ArrayList;

@SuppressWarnings("unchecked")
public class Solution {
    public String longestSubsequenceRepeatedK(String s, int k) {
        char[] ca = s.toCharArray();
        char[] freq = new char[26];
        for (char value : ca) {
            ++freq[value - 'a'];
        }
        ArrayList<String>[] cand = new ArrayList[8];
        cand[1] = new ArrayList<>();
        String ans = "";
        for (int i = 0; i < 26; i++) {
            if (freq[i] >= k) {
                ans = "" + (char) ('a' + i);
                cand[1].add(ans);
            }
        }
        for (int i = 2; i < 8; i++) {
            cand[i] = new ArrayList<>();
            for (String prev : cand[i - 1]) {
                for (String c : cand[1]) {
                    String next = prev + c;
                    if (isSubsequenceRepeatedK(ca, next, k)) {
                        ans = next;
                        cand[i].add(ans);
                    }
                }
            }
        }
        return ans;
    }

    private boolean isSubsequenceRepeatedK(char[] ca, String t, int k) {
        char[] ta = t.toCharArray();
        int n = ca.length;
        int m = ta.length;
        int i = 0;
        while (k-- > 0) {
            int j = 0;
            while (i < n && j < m) {
                if (ca[i] == ta[j]) {
                    j++;
                }
                i++;
            }
            if (j != m) {
                return false;
            }
        }
        return true;
    }
}

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