hdu 1392 Surround the Trees

#include<cstdio>
#include<cstring>
#include<iostream>
#include<string>
#include<map>
#include<algorithm>
#include<cmath>
using namespace std;

const int N=105;
const double eps=1e-8;

struct point{
double x;
double y;
}p
, stack
;

bool isZero(double x){
return (x>0 ? x : -x) <eps;
}

double dis(point A,point B){
return sqrt((A.x-B.x)*(A.x-B.x)+(A.y-B.y)*(A.y-B.y));
}

double crossProd(point A,point B,point C){
return (B.x-A.x)*(C.y-A.y)-(B.y-A.y)*(C.x-A.x);
}

int cmp(const void *a, const void *b) {
point *c = (point *)a;
point *d = (point *)b;
double k = crossProd(p[0], *c, *d);//极角大小转化为求叉乘
if (k<eps || isZero(k) && dis(p[0], *c)>dis(p[0], *d)) return 1;
return -1;
}

double Graham(int n)
{
int i,x=p[0].x,y=p[0].y;
int mi=0;
for( i=1;i<n;i++){
if(p[i].x<x||(p[i].x==x&&p[i].y<y)){
x=p[i].x;
y=p[i].y;
mi=i;
}
}
point tmp=p[mi];
p[mi]=p[0];
p[0]=tmp;
qsort(p+1, n-1, sizeof(point), cmp);
p
=p[0];
stack[0]=p[0];stack[1]=p[1];stack[2]=p[2];
int top=2;
for(i=3;i<=n;i++){
while(crossProd(stack[top-1],stack[top],p[i])<eps &&top>=2) --top;
stack[++top]=p[i];
}
double len=0;
for(int i=0;i<top;i++) len+=dis(stack[i],stack[i+1]);
return len;
}

int main()
{
int n;
while(scanf("%d",&n),n){
int i;
for(i=0;i<n;i++){
scanf("%lf %lf",&p[i].x,&p[i].y);
}
if(n==1) printf("0.00\n");
else if(n==2) printf("%.2lf\n",dis(p[0],p[1]));
else
printf("%.2lf\n",Graham(n));
}
}