【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
1 #include <stdio.h> 2 3 #define MAXNUM 105 4 5 typedef struct { 6 double x1, y1; 7 double x2, y2; 8 } line_st; 9 10 line_st lines[MAXNUM]; 11 12 int cal(int i, int j) { 13 double ab_x, ab_y, ac_x, ac_y, ad_x, ad_y; 14 double a, b; 15 16 ab_x = lines[i].x2 - lines[i].x1; 17 ab_y = lines[i].y2 - lines[i].y1; 18 ac_x = lines[j].x1 - lines[i].x1; 19 ac_y = lines[j].y1 - lines[i].y1; 20 ad_x = lines[j].x2 - lines[i].x1; 21 ad_y = lines[j].y2 - lines[i].y1; 22 a = ab_x*ac_y - ab_y*ac_x; 23 b = ab_x*ad_y - ab_y*ad_x; 24 if (a*b <= 0) 25 return 1; 26 else 27 return 0; 28 } 29 30 int main() { 31 int n; 32 int i, j, k; 33 34 while (scanf("%d", &n)!=EOF && n) { 35 for (i=0; i<n; ++i) 36 scanf("%lf%lf%lf%lf", &lines[i].x1,&lines[i].y1,&lines[i].x2,&lines[i].y2); 37 k = 0; 38 for (i=0; i<n; ++i) 39 for (j=0; j<i; ++j) 40 if (cal(i, j) && cal(j, i)) 41 k++; 42 printf("%d\n", k); 43 } 44 45 return 0; 46 }
