👨‍🏫 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 apublic 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);}
}