hdu 4667 Building Fence（二维凸包 Graham）

据说可以将圆划分成连续的小线段，大概250个差不多了

关键是二维凸包的Graham算法

#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;
const int MAXN = 50100;
const int cut = 250;
const double eps = 1e-8;
double pi,pc;
inline int zero(double a)
{
    if (a > eps) return 1;
    if (a < -eps) return -1;
    return 0;
}
struct _POINT
{
    double x, y;
    int cent;
    _POINT (){};
    _POINT (double a, double b, int c = -1) {x = a; y = b; cent=c;}
    double operator ^ (const _POINT & exa)
    {
        return x*exa.y - y*exa.x;
    }
    _POINT operator - (const _POINT & a)
    {
        return _POINT(x - a.x, y - a.y);
    }
}pt[MAXN];
inline double dis(const _POINT & a, const _POINT & b)
{
    return sqrt((a.x-b.x)*(a.x-b.x) + (a.y-b.y)*(a.y-b.y));
}
int npt, n, m, top, stk[MAXN];
double rr[3][MAXN];
bool cmp(_POINT a, _POINT b)
{
    double cax = (a - pt[0]) ^ (b - pt[0]);
    if (zero(cax) > 0) return true;
    if (zero(cax) < 0) return false;
    if (zero(dis(pt[0], a) - dis(pt[0], b)) < 0) return true;
    return false;
}
void Graham()
{
    _POINT p0 = pt[0];
    int k = 0;
    for (int i = 1; i< npt; ++i)
    {
        if (pt[i].y == p0.y)
        {
            if (pt[i].x < p0.x) k = i, p0 = pt[i];
        }
        else if (pt[i].y < p0.y) k = i, p0 = pt[i];
    }
    swap(pt[0], pt[k]);
    sort(pt+1, pt+npt, cmp);
    stk[0] = 0; stk[1] = 1;
    top = 2;
    for (int i = 2; i< npt; ++i)
    {
        while (top > 1 && zero((pt[stk[top-1]]-pt[stk[top-2]]) ^ (pt[i]-pt[stk[top-1]])) <= 0)
            top--;
        stk[top++] = i;
    }
}
double solve()
{
    double res = 0.0;
    stk[top++] = 0;
    for (int i = 1; i< top; ++i)
    {
        int u = stk[i-1], v = stk[i];
        if (pt[u].cent == pt[v].cent)
        {
            res += rr[2][pt[v].cent]*pi/cut;
        }
        else
        {
            res += dis(pt[u], pt[v]);
        }
    }
    return res;
}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
#endif    
    pi = acos(-1.0)*2.0; pc = pi/cut;
    while (scanf("%d%d", &n, &m) != EOF)
    {
        npt = 0;
        for (int i = 0; i< n; ++i)
        {
            double x, y, r, t = 0.0, xx, yy;
            scanf("%lf%lf%lf", &x, &y, &r);
            rr[0][i] = x;rr[1][i] = y;rr[2][i] = r;
            for (int j = 0; j< cut; ++j, t+=pc)
            {
                xx = x + r*cos(t); yy = y + r*sin(t);
                _POINT pp(xx, yy, i);
                pt[npt] = pp;
                npt++;
            }
        }
        for (int i = 0; i< m; ++i)
        {
            for (int j = 0; j< 3; ++j)
            {
                double x, y;
                scanf("%lf%lf", &x, &y);
                _POINT pp (x, y, npt);
                pt[npt] = pp;
                npt++;
            }            
        }
        Graham();
        printf("%.5lf\n", solve());
    }
    return 0;
}