#include <stdio.h>   
#include <algorithm>  
#define n 5;
int cmp(const void *a , const void *b)
{     
    return *(double *)a - *(double *)b;     
}   
   
double greedy(double a[],int n)
{   
    qsort(a,n,sizeof(double),cmp);   
    int mid = (n - 1) / 2;   
    double x[n];  
    x[mid] = a[n-1];   
    for(int i = mid+1;i < n;i++)   
        x[i] = a[n - 2*(i-mid)];   
    for(i = mid-1;i >= 0;i--)   
        x[i] = a[n - 2*(mid-i) - 1];   
    double sum = 0,exp = 0;   
    for(i = 0;i < n;i++)
    {   
        sum += a[i];   
        for(int j = i+1;j < n;j++)   
            exp+=x[i]*x[j]*(j-i);   
    }   
    return exp/sum/sum;   
}   
            
void main()
{   
    int i,j;   
    double a[1000],exp;   
    for(i = 0;i < n;i++)   
        scanf("%lf",&a[i]);   
    exp = greedy(a,n);   
    printf("%lf\n",exp);     
}  


为啥不能运行呃~~