开始复健。
用了不方便的算面积的方法，讨论了好多，还调了一个多小时- -
发现 double 和 longdouble 的四舍五入是随机的，有时候舍5有时候入5。
#include<cstdio> #include<algorithm> #include<cmath> using namespace std; long double x[3],y[3],x2,y2,tx,ty,dc; double ans; int bj=-1; bool fx; int main() { for(int i=0;i<3;i++) scanf("%Lf%Lf",&x[i],&y[i]); for(int i=0;i<3;i++) { int d=i+1; if(d==3)d=0; /*printf("i=%d d=%d (%.1Lf,%.1Lf) (%.1Lf,%.1Lf)\n", i,d,x[i],y[i],x[d],y[d]);*/ x2=x[i]-x[d]; if(x2<0)x2=-x2; y2=y[i]-y[d]; if(y2<0)y2=-y2; dc+=x2*y2; /*printf(" x1=%.1Lf y1=%.1Lf dc=%.1Lf\n",x2,y2,dc); printf(" tx=%.1Lf ty=%.1Lf\n",tx,ty);*/ if(i==0)tx=x2,ty=y2,bj=2,fx=1; else { if(x2>tx&&y2>ty) { bj=(i+2)%3; fx=1; tx=x2; ty=y2; } if(x2>tx)tx=x2,fx=0; if(y2>ty)ty=y2,fx=0; //printf("i=%d d=%d fx=%d\n",i,d,fx); } } dc/=2; //printf("bj=%d fx=%d\n",bj,fx); if(bj!=-1&&fx) { double tmp=0; /*for(int i=0;i<3;i++) { printf("x[%d]=%.2Lf y[%d]=%.2Lf\n", i,x[i],i,y[i]); } printf("bj=%d\n",bj);*/ if(bj!=1) { swap(x[bj],x[1]); swap(y[bj],y[1]); } /*for(int i=0;i<3;i++) { printf("x[%d]=%.2Lf y[%d]=%.2Lf\n", i,x[i],i,y[i]); }*/ if((x[0]-x[2])*(y[0]-y[2])>=0) { if((y[0]-y[1])/(x[0]-x[1])<(y[0]-y[2])/(x[0]-x[2])) tmp=abs((x[1]-x[2])*(y[0]-y[1])); else tmp=abs((x[0]-x[1])*(y[1]-y[2])); } else { if((y[0]-y[1])/(x[0]-x[1])<(y[0]-y[2])/(x[0]-x[2])) tmp=abs((x[0]-x[1])*(y[1]-y[2])); else tmp=abs((x[1]-x[2])*(y[0]-y[1])); } dc+=tmp; } //printf("tx=%.1Lf ty=%.1Lf\n",tx,ty); ans=tx*ty-dc; /*for(int i=0;i<3;i++) { int d=i+1; if(d==3)d=0; long double px=x[i]-x[d]; if(px<0)px=-px; long double py=y[i]-y[d]; if(py<0)py=-py; if(tx<px||ty<py) { printf("NO\n"); printf("x1=%Lf y1=%Lf px=%Lf py=%Lf\n", x1,y1,px,py); return 0; } } printf("YES\n");*/ int d=floor(ans*1000); d%=10; if(d==5)ans+=0.001; printf("%.2lf\n",ans); return 0; }