C++ 解法, 执行用时: 8ms, 内存消耗: 1528KB, 提交时间: 2019-11-21

#include <bits/stdc++.h>
#define LL long long
#define pb push_back
#define endl "\n"
#define FIN freopen("1.in","r",stdin)
#define mem(x,v) memset(x,v,sizeof(x))
#define rep(i,a,n) for(int i=a;i<n;i++)
#define per(i,a,n) for(int i=n-1;i>=a;i--)
using namespace std;
const int N = 26;
LL n,k,ans;
string s;
LL ch[N];
int main()
{
    ios::sync_with_stdio(false);
    while(cin>>n>>k>>s){
        ans = 0;
        mem(ch,0);
        rep(i,0,n){
            ch[s[i]-'A']++;
        }
        sort(ch,ch+N,greater<LL>());
        LL r = 0;
        while(k){
            if(k>ch[r]){
                ans+=ch[r]*ch[r];
                k-=ch[r];
                r++;
            }else{
                ans+=k*k;
                k = 0;
            }
 
        }
        cout<<ans<<endl;
    }
    return 0;
}

C++ 解法, 执行用时: 8ms, 内存消耗: 3640KB, 提交时间: 2020-07-28

#include <bits/stdc++.h>
#define LL long long
#define pb push_back
#define endl "\n"
#define FIN freopen("1.in","r",stdin)
#define mem(x,v) memset(x,v,sizeof(x))
#define rep(i,a,n) for(int i=a;i<n;i++)
#define per(i,a,n) for(int i=n-1;i>=a;i--)
using namespace std;
const int N = 26;
LL n,k,ans;
string s;
LL ch[N];
int main()
{
    ios::sync_with_stdio(false);
    while(cin>>n>>k>>s){
        ans = 0;
        mem(ch,0);
        rep(i,0,n){
            ch[s[i]-'A']++;
        }
        sort(ch,ch+N,greater<LL>());
        LL r = 0;
        while(k){
            if(k>ch[r]){
                ans+=ch[r]*ch[r];
                k-=ch[r];
                r++;
            }else{
                ans+=k*k;
                k = 0;
            }
  
        }
        cout<<ans<<endl;
    }
    return 0;
}