提交记录 代码

#include <bits/stdc++.h>
using namespace std;
const int maxn = 5e5 + 5;
int sum[maxn];
int main() {
	int k;
	string s;
	char c1, c2;
	cin >> k >> s >> c1 >> c2;
    int l = s.size();
	for (int i = l - 1; i >= 0; i--) {
		sum[i] = sum[i + 1];
		if (s[i] == c2)
			sum[i]++;
	}
	int cnt = 0;
	for (int i = 0; i < l; i++) 
		if (s[i] == c1) 
			cnt += sum[i + k - 1];
	cout << cnt;
	return 0;
}