CodeForces 659D Bicycle Race(叉积)
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题意:一个图,给你一个多边形,然后有一个人在上面按照顺时针去走,问你有多少条线段,如果他一直走,就会走到这个多边形的内部去。
思路:这个人在顺时针走,如果连着的两条线段,是逆时针的,显然这个人就走到多边形内部去了,用叉积去判一判就好了。
#include <cstdio>#include <queue>#include <cstring>#include <iostream>#include <cstdlib>#include <algorithm>#include <vector>#include <map>#include <string>#include <set>#include <ctime>#include <cmath>#include <cctype>using namespace std;#define LL long longint cas=1,T;const int maxn = 1e3+10;pair<int,int>p[maxn];bool check(pair<int,int>a,pair<int,int>b,pair<int,int>c){ return (b.first-a.first)*(c.second-b.second)-(c.first-b.first)*(b.second-a.second)>0;}int main(){int n,ans=0;scanf("%d",&n);n++;for (int i = 1;i<=n;i++)scanf("%d%d",&p[i].first,&p[i].second);for (int i = 3;i<=n;i++) if (check(p[i-2],p[i-1],p[i]))ans++;printf("%d\n",ans);//freopen("in","r",stdin);//scanf("%d",&T);//printf("time=%.3lf",(double)clock()/CLOCKS_PER_SEC);return 0;}
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