精选一百道题备赛蓝桥杯 —— 1.子串简写

前缀和与滑动窗口相结合

#include <iostream>
#include <string>
using namespace std;
long long a[500010];
char l, r;
int main()
{
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  long long ans = 0, cnt = 0;
  int k; cin >> k;
  string s; cin >> s >> l >> r;
  for(int i = 0; i <= s.length() - k; i++){
      if(s[i] == l) cnt++;
      if(s[i + k -1] == r) ans += cnt;
  }
  cout << ans;
  return 0;
}