同端子串的数量

/**
 * Problem: Leetcode 2955 - Count Equal Substrings
 *
 * Idea: Using Java Streams, we can achieve the same goal by creating a stream
 * of all possible substrings and then filtering those that have the same beginning
 * and ending characters. This solution leverages streams and lambda expressions,
 * providing a more functional approach.
 */

import java.util.stream.IntStream;

public class CountEqualSubstringsStream {
    public long countEqualSubstrings(String s) {
        int n = s.length();

        return IntStream.range(0, n)
                .mapToLong(i -> IntStream.range(i, n)
                        .filter(j -> s.charAt(i) == s.charAt(j))
                        .count())
                .sum();
    }

    public static void main(String[] args) {
        CountEqualSubstringsStream cess = new CountEqualSubstringsStream();
        String s = "abcab";
        System.out.println(cess.countEqualSubstrings(s)); // Output should be 7
    }
}