BZOJ3680 JSOI2004 平衡点 - 随机/近似算法

迭代乱搞了下就过了……

#include <bits/stdc++.h>
using namespace std;

double x[],y[],w[];
double xm,ym,wt,k,lambda=0.9;
int n;

const double ox[]={,,,-,},oy[]={,,,,-};

double calc(double xw,double yw){
    double ans=;
    for(int i=;i<=n;i++)
        ans+=sqrt((xw-x[i])*(xw-x[i])+(yw-y[i])*(yw-y[i]))*w[i];
    return ans;
}

int main() {
    cin>>n;
    for(int i=;i<=n;i++)
        cin>>x[i]>>y[i]>>w[i],
        xm+=x[i]*w[i],
        ym+=y[i]*w[i],
        k=max(k,max(abs(x[i]),abs(y[i]))),
        wt+=w[i];
    xm/=wt;
    ym/=wt;

    k*=0.1;

    double z=,oz=-1e+;

    while(k>=0.0001) {
        //printf("%.3f %.3f  k=%.4f\n",xm,ym,k);
        oz=z;
        double minopt=1e+;
        int opt=;double tmp;
        for(int i=;i<=;i++) {

            tmp=calc(xm+ox[i]*k,ym+oy[i]*k);
            if(tmp<minopt)
                minopt=tmp, opt=i;
        }
        xm+=ox[opt]*k, ym+=oy[opt]*k;
        z=tmp;
        k*=lambda;
    }

    printf("%.3f %.3f\n",xm,ym);
}