思路

滑动窗口 + 前缀和

过程

代码

#include <iostream>
#include <string>
#include <cstring>

using namespace std;

const int N = 2e5 + 10;

int n, l, r;
string s;

long long find(int x)
{
    int l = 0, r = 0, kind = 0, h[26] = { 0 };
    long long ans = 0;
    while(r < n)
    {
        if(h[s[r] - 'a'] ++ == 0) kind ++;
        while(kind > x) if(-- h[s[l ++] - 'a'] == 0) kind --;
        ans += r - l + 1;
        r ++;
    }
    return ans;
}

int main()
{
    cin >> n >> l >> r >> s;
    cout << find(r) - find(l - 1) << endl;
    return 0;
}