统计完全子字符串

/*
 * Problem: Find the number of fully matching substrings in the given string `word` where each character appears exactly `k` times,
 * and adjacent characters in the substring differ in their alphabetical positions by at most 2.
 *
 * Approach:
 * 1. Generate all substrings using IntStream.
 * 2. Use a frequency array to count the occurrences of each character in the substring.
 * 3. Check if each character in the substring appears exactly `k` times.
 * 4. Verify that the difference in the positions of adjacent characters is at most 2.
 * 5. Count the valid substrings.
 */
import java.util.stream.IntStream;

public int countFullyMatchingSubstringsStream(String word, int k) {
    int n = word.length();
    return (int) IntStream.range(0, n).flatMap(i -> IntStream.range(i + 1, n + 1)
            .mapToObj(j -> word.substring(i, j)))
            .filter(s -> {
                int[] freq = new int[26];
                s.chars().forEach(c -> freq[c - 'a']++);
                if (IntStream.of(freq).anyMatch(count -> count != 0 && count != k))
                    return false;
                return IntStream.range(1, s.length()).allMatch(i -> Math.abs(s.charAt(i) - s.charAt(i - 1)) <= 2);
            }).count();
}