又是一道细节题？
按题意一轮一轮分组就行了，最后一轮可能分不满，反正贼麻烦= =

#include <bits/stdc++.h>
using namespace std;
const int N=2501;
string s;
int n,a[N];

class EllysRoomAssignmentsDiv1 {
    public:
    double getAverage(vector <string> ra) {
        n=0;
        s="";
        for(int i=0;i<ra.size();i++) s+=ra[i];
        stringstream ss;
        ss<<s;
        while(ss>>a[n++]);
        n--;
        int Elly=a[0],w;
        int r=n/20;
        if (n%20) r++;
        sort(a,a+n,greater<int>());
        for(int i=0;i<n;i++) if (a[i]==Elly) w=i;
        double sum=0;
        for(int i=0;i+r-1<n;i+=r){
            if (i/r==w/r) continue;
            for(int j=i;j<i+r;j++) sum+=1.0*a[j]/r;
        }
        if (n%r==0) return (sum+Elly)/(n/r);
        if (w>=n/r*r) return (sum+Elly)/(n/r+1);
        double ans=sum;
        for(int i=n/r*r;i<n;i++) ans+=1.0*a[i]/(n%r);
        ans=(ans+Elly)/(n/r+1);
        sum=(sum+Elly)/(n/r);
        return ans*(n%r)/r+sum*(r-n%r)/r;
    }
};