51Nod 1098 最小方差 (数论)

 #include <iostream>
 #include <cstdio>
 #include <algorithm>
 using namespace std;

 typedef long long LL;
 const int MAXN = 1e4+;
 const LL INF = 1e15+;
 LL a[MAXN], sum[MAXN], ans[MAXN];

 int main()
 {
     int n, m;
     while(cin>>n>>m)
     {
         for(int i=; i<=n; i++)
             scanf("%lld",&a[i]);
         sort(a+, a++n);
         sum[] = ans[] = ;
         for(int i=; i<=n; i++){
             sum[i] = sum[i-] + a[i];           //前缀和
             ans[i] = ans[i-] + a[i]*a[i];      //^2的前缀和
         }
         double Min = INF;
         for(int i=m; i<=n; i++){
             double tmp = (ans[i]-ans[i-m])-1.0*(sum[i]-sum[i-m])*(sum[i]-sum[i-m]) / m;
             if(tmp < Min)
                 Min = tmp;
         }
         printf("%lld\n", (LL)Min);　　//强行转不要小数点后面，防止四舍五入
     }
     return ;
 }