HDU 1392 Surround the Trees

题解：计算凸包……

#include <cstdio>  

#include <cstring>  

#include <algorithm>  

#include <cmath>

using namespace std;  

struct node{int x,y;}vex[1005],stackf[1005]; 

bool cmp1(node a,node b){  

    if(a.y==b.y)return a.x<b.x;  

    else return a.y<b.y;  

}  

int cross(node a,node b,node c){return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y);} 

double dis(node a,node b){return sqrt((a.x-b.x)*(a.x-b.x)*1.0+(a.y-b.y)*(a.y-b.y));} 

bool cmp2(node a,node b){  

    int m=cross(vex[0],a,b);  

    if(m==0)return dis(vex[0],a)-dis(vex[0],b)<=0?1:0;  

    else return m>0?1:0;  

}  

int main(){  

    int t;  

    while(scanf("%d",&t),t!=0){  

        for(int i=0;i<t;i++)scanf("%d%d",&vex[i].x,&vex[i].y);  

        if(t==1)printf("%.2f\n",0.00);  

        else if(t==2)printf("%.2f\n",dis(vex[0],vex[1]));  

        else{  

            sort(vex,vex+t,cmp1);  

            sort(vex+1,vex+t,cmp2);  

            memset(stackf,0,sizeof(stackf));  

            stackf[0]=vex[0]; stackf[1]=vex[1];  

            int top=1;  

            for(int i=2;i<t;i++){  

                while(i>=1&&cross(stackf[top-1],stackf[top],vex[i])<0)top--;      

                stackf[++top]=vex[i];   

        }  

        double s=0;  

        for(int i=1;i<=top;i++)s+=dis(stackf[i-1],stackf[i]);  

        s+=dis(stackf[top],vex[0]);  

        printf("%.2f\n",s);  

        }  

    }  

    return 0;

}