poj 2242（水题，注意分情况讨论）

#include<iostream>
#define PI 3.141592653589793
#include<cmath>
using namespace std;
int main(){
    double x1,y1,x2,y2,x3,y3;
    double k1,b1,k2,b2,x,y,result;
    while(scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3)==6){
        if(x1==x2){
            y = (y1+y2)/2;
            if(y1==y3){
                x = (x1+x3)/2;
            }
            else{
                k2 = -(x3-x1)/(y3-y1); 
                b2 = (y1+y3)/2-k2*(x1+x3)/2;
                x = (y-b2)/k2;
            }
        }
        else{
            if(y1==y2){
                x = (x1+x2)/2;
                if(x1==x3){
                    y = (y1+y3)/2;
                }
                else{
                    k2 = -(x3-x1)/(y3-y1);
                    b2 = (y1+y3)/2-k2*(x1+x3)/2;
                    y = k2*x+b2; 
                }
            }
            else{
                k1 = -(x2-x1)/(y2-y1);
                b1 = (y1+y2)/2-k1*(x1+x2)/2;
                if(x1==x3){
                    y = (y1+y3)/2;
                    x = (y-b1)/k1;
                }
                else if(y1==y3){
                    x = (x1+x3)/2;
                    y = k1*x+b1;
                }
                else{
                    k2 = -(x3-x1)/(y3-y1);
                    b2 = (y1+y3)/2-k2*(x1+x3)/2;
                    x = -(b2-b1)/(k2-k1);
                    y = k1*x+b1;
                } 
            }
        }
        result = PI*pow((x-x1)*(x-x1)+(y-y1)*(y-y1),0.5)*2;
        printf("%.2lf\n",result);
    }
    return 0;
}