题解 | abb

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        //输入字符串
        String str = sc.next();

        //后缀和数组，suffix[i+1][j]表示str中第i个字母之后的对应字母出现次数
        int[][] suffix = new int[n + 1][26];
        for (int i = n - 1; i >= 0; i--) {
            char c = str.charAt(i);
            for (int j = 0; j < 26; j++) {
                suffix[i][j] = suffix[i + 1][j];
            }
            suffix[i][c - 'a']++;
        }

        //记录所有"abb"型子序列的个数
        long res = 0;

        for (int i = 0; i < n; i++) {
            char c = str.charAt(i);
            for (int j = 0; j < 26; j++) {
                //加上当前字母为前缀，所组成的"abb"型子序列的个数
                if (j != c - 'a' && suffix[i][j] >= 2) {
                    res += suffix[i + 1][j] * (suffix[i + 1][j] - 1) / 2;
                }
            }
        }


        System.out.println(res);
    }
}