长度为 K 的无重复字符子串

/*
 * Problem: Find all substrings of length K with no repeating characters using Java Streams.
 * Approach:
 * 1. Generate all possible substrings of length K using IntStream.
 * 2. Filter substrings that have unique characters by checking their set size.
 */
import java.util.*;
import java.util.stream.*;

public class Solution {
    public List<String> numKLenSubstrNoRepeats(String S, int K) {
        if (S.length() < K) return Collections.emptyList();
        return IntStream.rangeClosed(0, S.length() - K)
                        .mapToObj(i -> S.substring(i, i + K))
                        .filter(sub -> sub.chars().distinct().count() == K)
                        .collect(Collectors.toList());
    }
}