HDU 1392 Surround the Trees 构造凸包

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <queue>
#include <cmath>
#include <algorithm>

using namespace std;

struct P
{
int x,y;
double angle;
} p[50010];

bool cmp(P p1,P p2)
{
return p1.angle > p2.angle;
}

double calangle(P p,P t)
{
return ((t.x-p.x)*(t.x-p.x) + 1 - (t.x-p.x-1)*(t.x-p.x-1))/(2*sqrt((t.x-p.x)*(t.x-p.x) + (t.y-p.y)*(t.y-p.y)));
}

P sp[50010];

bool judgesite(P p1,P p2, P p)
{
P l1,l2;
l1.x = p2.x - p1.x;
l1.y = p2.y - p1.y;

l2.x = p.x - p1.x;
l2.y = p.y - p1.y;

return (l1.x*l2.y - l1.y*l2.x) > 0 ? true : false ;
}

double callen(P p1,P p2)
{
return sqrt( (p1.x-p2.x)*(p1.x-p2.x) + (p1.y-p2.y)*(p1.y-p2.y)   );
}

int main()
{
int n,i;

int top;

double MinLen;

while(scanf("%d",&n) && n)
{

top = 0;

for(i = 1; i <= n; ++i)
{
scanf("%d %d",&p[i].x,&p[i].y);
}

P temp;

for(temp = p[1],i = 2; i <= n; ++i)
{
if(temp.y > p[i].y)
{
temp = p[i];
}
else if(temp.y == p[i].y && temp.x > p[i].x)
{
temp = p[i];
}
}

for(i = 1; i <= n; ++i)
{
if(temp.x != p[i].x || temp.y != p[i].y)
{
p[i].angle = calangle(temp,p[i]);
}
else p[i].angle = -2;
}

sort(p+1,p+n+1,cmp);

sp[top++] = p
;
sp[top++] = p[1];

for(i = 2; i <= n;)
{
if(judgesite(sp[top-2],sp[top-1],p[i]))
{
sp[top++] = p[i];
++i;
}
else top--;
}

top--;//最后一个点重复了

MinLen = callen(sp[top-1],sp[0]);

for(i = 1; i < top; ++i)
{
MinLen += callen(sp[i-1],sp[i]);
}

printf("%.2lf\n",MinLen);
}
return 0;
}