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

 1 #include<stdio.h>
 2 #include<iostream>
 3 #include<algorithm>
 4 #include<math.h>
 5 #include<string.h>
 6 #include<string>
 7 #include<map>
 8 #include<set>
 9 #include<vector>
10 #include<queue>
11 #define M(a,b) memset(a,b,sizeof(a))
12 using namespace std;
13 typedef long long ll;
14 struct point{
15     int x,y;
16 }pnt[1005];
17 int solve(point a,point b,point c){
18     return (b.x-a.x)*(c.y-b.y)-(c.x-b.x)*(b.y-a.y);
19 }
20 int main(){
21     int n;
22     scanf("%d",&n);
23     n++;
24     for(int i=0;i<n;i++)
25         scanf("%d%d",&pnt[i].x,&pnt[i].y);
26     int ans=0;
27     for(int i=2;i<n;i++)
28         if(solve(pnt[i-2],pnt[i-1],pnt[i])>0) ans++;
29     printf("%d\n",ans);
30     return 0;
31 }
32 /*
33 
34 6
35 0 0
36 0 1
37 1 1
38 1 2
39 2 2
40 2 0
41 0 0
42 
43 16
44 1 1
45 1 5
46 3 5
47 3 7
48 2 7
49 2 9
50 6 9
51 6 7
52 5 7
53 5 3
54 4 3
55 4 4
56 3 4
57 3 2
58 5 2
59 5 1
60 1 1
61 
62 */