HDU 1392 Surround the Trees 构造凸包

又是一道模板题

#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[n];
        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;
}