BZOJ 2428 [HAOI2006]均分数据 模拟退火

#include <cstdio>
#include <cmath>
#include <algorithm>
#define N 25
#define INF 1e20
using namespace std;
inline int RAND() { return /*(rand()<<15)+*/rand(); }
inline double sq(double x) { return x*x; }
int n,m,b
;
double a
,sum
,ave,ans=INF;
double SAA() { //Simulate Anneal Arithmetic
for(int i=1;i<=m;++i) sum[i]=0;
for(int i=1;i<=n;++i)
b[i]=RAND()%m+1, sum[b[i]]+=a[i];
double T=1e5,last,now=0;
for(int i=1;i<=m;++i) now+=sq(sum[i]-ave);
for(;T>1e-5;T*=0.75) {
last=now;
int x=RAND()%n+1,y=0;
if(T<=1e4) y=rand()%m+1;
for(int i=1;i<=m;++i)
if(sum[i]<sum[y]) y=i;
now+=a[x]*(a[x]+sum[y]-sum[b[x]])*2;
if(now>last && T<RAND()%100000) now=last;
else sum[b[x]]-=a[x], sum[y]+=a[x], b[x]=y;
}
return now;
}
int main() {
srand(20000301);
scanf("%d%d",&n,&m);
for(int i=1;i<=n;++i) scanf("%lf",a+i), ave+=a[i];
sum[0]=INF, ave/=m;
for(int i=1;i<=50000;++i) ans=min(ans,SAA());
printf("%.2f\n",sqrt(ans/m));
return 0;
}