HDU 1392 Surround the Trees

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define _DEBUG 1
#define MAX_TREE 110

typedef struct {
double x,y;
}Point;
Point pts[MAX_TREE];
Point chs[MAX_TREE];
inline void exchange(Point &pa,Point &pb){
Point tmp = pa;
pa = pb;
pb = tmp;
}

double dist(Point pa,Point pb){
return sqrt((pa.x-pb.x)*(pa.x-pb.x)+(pa.y-pb.y)*(pa.y-pb.y));
}

double crossMultiple(Point p0,Point p1,Point p2){
return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}

int compare(const void *p1,const void *p2){
Point p3 = *(Point *)p1;
Point p4 = *(Point *)p2;

double cm = crossMultiple(pts[0],p3,p4);
if(cm < 0)
return 1;
else if(cm == 0 && dist(pts[0],p3) > dist(pts[0],p4))
return 1;
else
return -1;
}

double convexHull(int n){
int i;
int tmp=0;
double sum = 0;

for(i=1;i<n;++i){//寻找第0个点
if(pts[i].y < pts[tmp].y)
tmp = i;
else if(fabs(pts[i].y-pts[tmp].y)<0.000001){//不严格
if(pts[i].x < pts[tmp].x)
tmp=i;
}
}
exchange(pts[tmp],pts[0]);

qsort(pts+1,n-1,sizeof(Point),compare);//排序,注意不包含第0个

chs[0] = pts[0];
chs[1] = pts[1];
int top = 1;
for(i=2;i<n;++i){
while(top>=1 && crossMultiple(chs[top-1],chs[top],pts[i]) <= 0)
top--;
chs[++top] = pts[i];
}
chs[++top] = chs[0];
for(i=0;i<top;i++){
sum += dist(chs[i],chs[i+1]);
}
return sum;
}
int main(){
int n;
int i;
#if _DEBUG==1
freopen("hdu1392.in","r",stdin);
#endif
while(scanf("%d",&n) && n!=0){
for(i=0;i<n;++i){
scanf("%lf %lf",&pts[i].x,&pts[i].y);
}
if(n==1){
printf("%.2f\n",0.00);
continue;
}
if(n==2){//注意两个点的情况：n=2的时候，输出那2个点的EuclidDis
printf("%.2lf\n",dist(pts[0],pts[1]));
continue;
}

double sum = convexHull(n);
printf("%.2lf\n",sum);
}
return 0;
}