【HDOJ】1086 You can Solve a Geometry Problem too

数学题，证明AB和CD。只需证明C、D在AB直线两侧，并且A、B在CD直线两侧。
公式为：（ABxAC）*（ABxAD）<= 0 and（CDxCA）*（CDxCB）<= 0

 #include <stdio.h>

 #define MAXNUM 105

 typedef struct {
     double x1,  y1;
     double x2,  y2;
 } line_st;

 line_st lines[MAXNUM];

 int cal(int i, int j) {
     double ab_x, ab_y, ac_x, ac_y, ad_x, ad_y;
     double a, b;

     ab_x = lines[i].x2 - lines[i].x1;
     ab_y = lines[i].y2 - lines[i].y1;
     ac_x = lines[j].x1 - lines[i].x1;
     ac_y = lines[j].y1 - lines[i].y1;
     ad_x = lines[j].x2 - lines[i].x1;
     ad_y = lines[j].y2 - lines[i].y1;
     a = ab_x*ac_y - ab_y*ac_x;
     b = ab_x*ad_y - ab_y*ad_x;
     if (a*b <= )
         return ;
     else
         return ;
 }

 int main() {
     int n;
     int i, j, k;

     while (scanf("%d", &n)!=EOF && n) {
         for (i=; i<n; ++i)
             scanf("%lf%lf%lf%lf", &lines[i].x1,&lines[i].y1,&lines[i].x2,&lines[i].y2);
         k = ;
         for (i=; i<n; ++i)
             for (j=; j<i; ++j)
                 if (cal(i, j) && cal(j, i))
                     k++;
         printf("%d\n", k);
     }

     return ;
 }