hdu 1392 Surround the Trees

凸包：

#include<stdio.h>
#include<math.h>
#include<stdlib.h>
struct node
{
int x,y;
};
int stack[101];
int top;
struct node point[101];
void swap(struct node point[],int x,int y) //交换两点
{
struct node temp;
temp=point[x];
point[x]=point[y];
point[y]=temp;
}

double distance(struct node p1,struct node p2) //两点间的距离
{
return sqrt(double((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y)));
}

double multi(struct node p1,struct node p2,struct node p0) //叉乘
{
return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}

int cmp(const void *a,const void *b)
{
struct node *c=(struct node*)a;
struct node *d=(struct node*)b;
double k=multi(*c,*d,point[0]);
if(k<=0)return 1; // k<=0 说名c在d的逆时针方向,相对于point[0]，c的极角比d的大
return -1; //如果 k>0 时返回1那么就会按 (p1.x-p0.x)*(p2.y-p0.y)从小到大排序，那么也
}        //就是按极角从大到小排序 ，应该要按极角从小到大排序，所以返回-1
void merge(int n)
{
int u=0;
int i;
for(i=0;i<n;i++)
if((point[i].y < point[u].y) || (point[i].y == point[u].y && point[i].x < point[u].x))
u=i;
swap(point,0,u);
qsort(point+1,n-1,sizeof(point[0]),cmp);
for(i=0;i<=2;i++)
stack[i]=i;
top=2;
for(i=3;i<n;i++)
{
while(multi(point[stack[top]],point[i],point[stack[top-1]])<=0)
top--;
//    top++;
stack[++top]=i;
}
}
double length(int n)
{
double sumlen=0;
int i;
for(i=0;i<=n-1;i++)
sumlen+=distance(point[stack[i]],point[stack[i+1]]);
sumlen+=distance(point[0],point[stack
]);
return sumlen;
}

int main()
{
int i,n;
while(scanf("%d",&n),n)
{
for(i=0;i<n;i++)
scanf("%d%d",&point[i].x,&point[i].y);
if(n<2)
{
printf("0.00\n");
continue;
}
if(n==2)
{
printf("%.2lf\n",distance(point[0],point[1]));
continue;
}
merge(n);
printf("%.2lf\n",length(top));
}
return 0;
}