思路

dp

过程

代码

#include <iostream>
#include <unordered_map>

using namespace std;

const int N = 1e5 + 10;

typedef long long LL;

int n;
string s;
LL h[26], ch[26];

int main()
{
    cin >> n >> s;
    LL ans = 0;
    for(int i = 0;i < n;i ++)
    {
        char x = s[i] - 'a';
        ans += h[x];
        h[x] = h[x] + i - ch[x];
        ch[x] ++;
    }
    cout << ans << endl;
    return 0;
}