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;
}