[JSOI2004]平衡点 / 吊打XXX
问题分析
随后系统的势能应当最低,即
\[\sum w_i \times \sqrt{(x-x_i)^2+(y-y_i)^2}
\]
最小。直接模拟退火。
参考程序
#include <cstdio>
#include <ctime>
#include <cstdlib>
#include <cmath>
#define Maxn 1010
int n, x[Maxn], y[Maxn], w[Maxn];
double X, Y, Cost, AnsX, AnsY, AnsC;
inline double Rand() { return (double)rand() / RAND_MAX; }
inline double Calc(double X, double Y) {
double Ans = 0;
for (int i = 1; i <= n; ++i)
Ans += sqrt((x[i] - X) * (x[i] - X) + (y[i] - Y) * (y[i] - Y)) * w[i];
if (Ans < AnsC) AnsX = X, AnsY = Y, AnsC = Ans;
return Ans;
}
int main() {
srand(time(NULL));
scanf("%d", &n);
for (int i = 1; i <= n; ++i) scanf("%d%d%d", &x[i], &y[i], &w[i]);
for (int i = 1; i <= n; ++i) AnsX += x[i], AnsY += y[i];
AnsX = X = AnsX / n;
AnsY = Y = AnsY / n;
Cost = AnsC = Calc(X, Y);
double T = 10000;
while (T > 0.001) {
double XX = X + T * (Rand() * 2 - 1);
double YY = Y + T * (Rand() * 2 - 1);
double C = Calc(XX, YY);
if (exp((Cost - C) / T) > Rand()) X = XX, Y = YY, Cost = C;
T *= 0.999;
}
for (int i = 1; i <= 1000; ++i) { //再来一点随机扰动
double XX = AnsX + T * (Rand() * 2 - 1);
double YY = AnsY + T * (Rand() * 2 - 1);
Calc(XX, YY);
}
printf("%.3lf %.3lf\n", AnsX, AnsY);
return 0;
}