hdu 1392【Surround the Trees】

凸包

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

struct node
{
    int x,y;
}p[1000],stack[1000];
int n,top;

double dis(node &a,node &b)
{
    return sqrt(1.0*(a.x - b.x) * (a.x - b.x) + 1.0 * (a.y-b.y) * (a.y - b.y));
}

double polar(node &a,node &b,node & o)
{
    return 1.0*(a.x - o.x) * (b.y - o.y) - 1.0*(b.x - o.x) * (a.y - o.y);
}

bool cmp(node& a,node& b)
{
    double temp = polar(a,b,p[0]);
    
    if(temp == 0)
    {
        return dis(a,p[0]) < dis(b,p[0]);
    }

    return temp > 0;
}

void convex_hull()
{
    stack[0] = p[0];
    stack[1] = p[1];
    top = 1;
    for(int i = 2;i < n;i ++)
    {
        while(top && polar(p[i],stack[top],stack[top-1]) >= 0)
        {
            top -- ;
        }
        stack[++top] = p[i];
    }
}

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

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

        double sum = 0;
        for(int i = 1;i <= top;i ++)
        {
            sum += dis(stack[i],stack[i-1]);
        }
        sum += dis(stack[0],stack[top]);
        printf("%.2lf\n",sum);
    }

    return 0;
}

可以当做模板来用