HDU 1392 Surround the Trees（凸包）

题面

懒得粘贴了。。。
大致题意：坐标系内有若干个点，问把这些点都圈起来的最小凸包周长。

题解

直接求出凸包，统计一遍答案即可

#include<iostream>#include<cstdio>#include<cstdlib>#include<cstring>#include<cmath>#include<queue>#include<algorithm>using namespace std;#define MAX 10000#define INF 1000000000int n,top;struct Node{       int x,y;}p[MAX],S[MAX];inline int read(){       int x=0,t=1;char ch=getchar();       while((ch<'0'||ch>'9')&&ch!='-')ch=getchar();       if(ch=='-'){t=-1;ch=getchar();}       while(ch>='0'&&ch<='9'){x=x*10+ch-48;ch=getchar();}       return x*t;}inline bool cmp(Node a,Node b){       double A=atan2((a.y-p[1].y),(a.x-p[1].x));       double B=atan2((b.y-p[1].y),(b.x-p[1].x));       if(A!=B)return A<B;       else    return a.x<b.x;}long long Cross(Node a,Node b,Node c){       return 1LL*(b.x-a.x)*(c.y-a.y)-1LL*(b.y-a.y)*(c.x-a.x);}void Get(){       p[0]=(Node){INF,INF};int k;       for(int i=1;i<=n;++i)              if(p[0].y>p[i].y||(p[0].y==p[i].y&&p[i].x<p[0].x))               {p[0]=p[i];k=i;}       swap(p[k],p[1]);       sort(&p[2],&p[n+1],cmp);       S[0]=p[1],S[1]=p[2];top=1;       for(int i=3;i<=n;)       {              if(top&&Cross(S[top-1],p[i],S[top])>=0)top--;              else  S[++top]=p[i++];       }}inline double Dis(Node a,Node b){       return sqrt(1LL*(a.x-b.x)*(a.x-b.x)+1LL*(a.y-b.y)*(a.y-b.y));}inline void GetAns(){       double Ans=0;       if(top==0)          Ans=0;       else       if(top==1)          Ans=Dis(S[0],S[1]);       else       {           S[++top]=S[0];           for(int i=0;i<top;++i)             Ans+=Dis(S[i],S[i+1]);       }       printf("%.2f\n",Ans);}int main(){       while(233333)       {              n=read();              if(!n)break;              for(int i=1;i<=n;++i)                 p[i]=(Node){read(),read()};              Get();              GetAns();        }       return 0;}