几何——BAPC2019 K

感觉这题思路简单，其实就是解一个二元一次方程组

#include<bits/stdc++.h>
using namespace std;
#define N 20

int x[N],y[N],n; 
double xa,xb,xc,ya,yb,yc,X,Y;

int main(){
    cin>>n>>X>>Y;
    for(int i=1;i<=n;i++)
        cin>>x[i]>>y[i];
    for(int i=1;i<=n;i++)
        for(int j=i+1;j<=n;j++)
            for(int k=j+1;k<=n;k++){
                double a,b,c;
                xa=x[i],ya=y[i];
                xb=x[j],yb=y[j];
                xc=x[k],yc=y[k];
                if(xa!=xc){
                    double tmp=Y-yc-(ya-yc)*(X-xc)/(xa-xc);
                    if(tmp==0){
                        b=0,a=(X-xc)/(xa-xc);
                    }else {
                        b=tmp/(yb-yc-(ya-yc)*(xb-xc)/(xa-xc));
                        a=(X-xc)/(xa-xc)-(xb-xc)/(xa-xc)*b;
                    }
                }else {//xa==xc
                    b=(X-xc)/(xb-xc);
                    a=(Y-yc-b*(yb-yc))/(ya-yc);
                }
                c=1-a-b;
                if(a>=0 && b>=0 && c>=0){
                    for(int l=1;l<=n;l++){
                        if(l==i)printf("%.10lf\n",a);
                        else if(l==j)printf("%.10lf\n",b);
                        else if(l==k)printf("%.10lf\n",c);
                        else cout<<0<<'\n';
                    }
                    return 0;
                }
            }
}