👨‍🏫 Acwing 字串简写

输入

4
abababdb a b

输出

6

💖 前缀和

import java.util.Scanner;

public class Main
{
	static int N = (int) 5e5 + 10;
	static int[] l = new int[N];// l[i] 表示 i 以及 i 左边包含多少个字符 a

	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);
		int k = sc.nextInt();
		String s = sc.next();
		s = "0" + s;//占位符，下标为 0 不用
		char a = sc.next().charAt(0);
		char b = sc.next().charAt(0);
		char[] ss = s.toCharArray();
		int n = ss.length;
		l[0] = 0;
		//处理前缀和数组
		for (int i = 1; i < n; i++)
		{
			l[i] = l[i - 1];
			if (ss[i] == a)
				l[i]++;
		}
		long ans = 0;
		for (int i = 0; i < n; i++)
			// 从第k个开始处理
			if (i >= k && ss[i] == b)
			{
				ans += l[i - k + 1];
			}
		System.out.println(ans);
	}
}