最长重复子串

/*
 * Problem: Find the longest duplicate substring in a given string using Java Streams.
 * 
 * Approach:
 * 1. Utilize binary search to determine the length of the longest duplicate substring.
 * 2. Use the Stream API and a hash set to find duplicates efficiently.
 * 3. The Rabin-Karp rolling hash algorithm is used for hashing substrings.
 */

import java.util.*;
import java.util.stream.*;

public class SolutionStream {
    public String longestDupSubstring(String s) {
        int left = 1, right = s.length();
        String result = "";
        while (left < right) {
            int mid = left + (right - left) / 2;
            String dup = findDuplicateUsingStream(s, mid);
            if (dup != null) {
                result = dup;
                left = mid + 1;
            } else {
                right = mid;
            }
        }
        return result;
    }

    private String findDuplicateUsingStream(String s, int length) {
        Set<Long> seen = new HashSet<>();
        long base = 26, mod = (long) Math.pow(2, 32);
        return IntStream.range(0, s.length() - length + 1)
                .mapToObj(i -> s.substring(i, i + length))
                .mapToLong(sub -> sub.chars().mapToLong(c -> c - 'a').reduce(0, (acc, c) -> (acc * base + c) % mod))
                .filter(hash -> !seen.add(hash))
                .mapToObj(hash -> s.substring(seen.stream().mapToInt(seen::hashCode).boxed().collect(Collectors.toList()).indexOf(hash),
                        seen.stream().mapToInt(seen::hashCode).boxed().collect(Collectors.toList()).indexOf(hash) + length))
                .findFirst().orElse(null);
    }
}