[ An Ac a Day ^_^ ] CodeForces 659D Bicycle Race 计算几何 叉积

问有多少个点在多边形内

求一遍叉积 小于零计数就好了~

 #include<stdio.h>
 #include<iostream>
 #include<algorithm>
 #include<math.h>
 #include<string.h>
 #include<string>
 #include<map>
 #include<set>
 #include<vector>
 #include<queue>
 #define M(a,b) memset(a,b,sizeof(a))
 using namespace std;
 typedef long long ll;
 struct point{
     int x,y;
 }pnt[];
 int solve(point a,point b,point c){
     return (b.x-a.x)*(c.y-b.y)-(c.x-b.x)*(b.y-a.y);
 }
 int main(){
     int n;
     scanf("%d",&n);
     n++;
     for(int i=;i<n;i++)
         scanf("%d%d",&pnt[i].x,&pnt[i].y);
     int ans=;
     for(int i=;i<n;i++)
         if(solve(pnt[i-],pnt[i-],pnt[i])>) ans++;
     printf("%d\n",ans);
     return ;
 }
 /*

 6
 0 0
 0 1
 1 1
 1 2
 2 2
 2 0
 0 0

 16
 1 1
 1 5
 3 5
 3 7
 2 7
 2 9
 6 9
 6 7
 5 7
 5 3
 4 3
 4 4
 3 4
 3 2
 5 2
 5 1
 1 1

 */