bzoj 2508: 简单题【拉格朗日乘数法】

大概是对于f(x,y)求min，先把x看成常数，然后得到关于y的一元二次方程，然后取一元二次极值把y用x表示，再把x作为未知数带回去化简，最后能得到一个一元二次的式子，每次修改这个式子的参数即可。
智商欠费解释不清，详见Claris大神 http://www.cnblogs.com/clrs97/p/4403197.html

#include<cstdio>
#include<cmath>
using namespace std;
const int N=120005;
int n,q,op,i,m;
double X1,X2,Y1,Y2,a,b,c,d,aa[N],bb[N],cc[N],ab[N],ac[N],bc[N],saa,sbb,scc,sab,sac,sbc,eps=1e-8,ans;
inline bool cmp(double x)
{
	return fabs(x)<eps;
}
inline double solve(double a,double b,double c)
{
	if(cmp(a))
		return c;
	double x=-b/(2.0*a);
	return a*x*x+b*x+c;
}
int main()
{
	scanf("%d",&q);
	while(q--)
	{
		scanf("%d",&op);
		if(op==0)
		{
			scanf("%lf%lf%lf%lf",&X1,&Y1,&X2,&Y2);
			if(cmp(X1-X2))
				a=1,b=0,c=-X1;
			else 
				a=(Y2-Y1)/(X2-X1),b=-1,c=Y1-a*X1;
			d=a*a+b*b;
			aa[++n]=a*a/d,bb[n]=b*b/d,cc[n]=c*c/d,ab[n]=a*b/d,ac[n]=a*c/d,bc[n]=b*c/d;
			saa+=aa[n],sbb+=bb[n],scc+=cc[n],sab+=ab[n],sac+=ac[n],sbc+=bc[n];
			m++;
		}
		if(op==1)
		{
			scanf("%d",&i);
			saa-=aa[i],sbb-=bb[i],scc-=cc[i],sab-=ab[i],sac-=ac[i],sbc-=bc[i];
			m--;
		}
		if(op==2)
		{
			if(!m)
			{
				puts("0.00");
				continue;
			}
			if(cmp(sbb))
				a=b=0;
			else 
				a=-sab/sbb,b=-sbc/sbb;
			ans=solve(saa+2.0*a*sab+a*a*sbb,2.0*(b*sab+sac+a*b*sbb+a*sbc),b*b*sbb+2.0*b*sbc+scc);
			if(cmp(ans))
				ans=0;
			printf("%.2f
",ans);
		}
	}
	return 0;
}