hoj1392

凸包问题，求出凸包后，计算距离，不知道为什么n==2的情况居然不是两点距离的2倍，而是正好是两点的距离。

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

#define maxn 505

struct point
{
    double x, y;
}pnt[maxn], res[maxn];

int n, m;

bool mult(point sp, point ep, point op)
{
    return (sp.x - op.x) * (ep.y - op.y)>= (ep.x - op.x) * (sp.y - op.y);
}

bool operator < (const point &l, const point &r)
{
    return l.y < r.y || (l.y == r.y && l.x < r.x);
}

void input()
{
    for (int i = 0;i < n; i++)
    {
        scanf("%lf%lf", &pnt[i].x, &pnt[i].y);
    }
}

int graham()
{
    int i, len, top = 1;
    sort(pnt, pnt + n);
    if (n == 0) return 0; res[0] = pnt[0];
    if (n == 1) return 1; res[1] = pnt[1];
    if (n == 2) return 2; res[2] = pnt[2];
    for (i = 2; i < n; i++)
    {
        while (top && mult(pnt[i], res[top], res[top-1]))
            top--;
        res[++top] = pnt[i];
    }
    len = top; res[++top] = pnt[n - 2];
    for (i = n - 3; i >= 0; i--)
    {
        while (top!=len && mult(pnt[i], res[top], res[top-1])) top--;
        res[++top] = pnt[i];
    }
    return top; // 返回凸包中点的个数
}

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

double work()
{
    double ret = dist(res[m - 1], res[0]);
    for (int i = 0; i < m - 1; i++)
        ret += dist(res[i], res[i + 1]);
    if (n == 2)
        return ret / 2;
    return ret;
}

int main()
{
    //freopen("D:\\t.txt", "r", stdin);
    while (scanf("%d", &n) != EOF && n != 0)
    {
        input();
        m = graham();
        printf("%.2f\n", work());
    }
    return 0;
}