hdu 3662 3D Convex Hull

Problem - 3662

　　题意很简单，构造三维凸包，求凸包有多少个面。

代码如下：

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

 using namespace std;

 const double EPS = 1e-;
 const int N = ;
 inline int sgn(double x) { return (x > EPS) - (x < -EPS);}
 struct Point {
     double x, y, z;
     Point() {}
     Point(double x, double y, double z) : x(x), y(y), z(z) {}
     bool operator < (Point a) const {
         if (sgn(x - a.x)) return x < a.x;
         if (sgn(y - a.y)) return y < a.y;
         return z < a.z;
     }
     bool operator == (Point a) const { return !sgn(x - a.x) && !sgn(y - a.y) && !sgn(z - a.z);}
     Point operator + (Point a) { return Point(x + a.x, y + a.y, z + a.z);}
     Point operator - (Point a) { return Point(x - a.x, y - a.y, z - a.z);}
     Point operator * (double p) { return Point(x * p, y * p, z * p);}
     Point operator / (double p) { return Point(x / p, y / p, z / p);}
 } ;
 inline double dot(Point a, Point b) { return a.x * b.x + a.y * b.y + a.z * b.z;}
 inline double dot(Point o, Point a, Point b) { return dot(a - o, b - o);}
 inline Point cross(Point a, Point b) { return Point(a.y * b.z - a.z * b.y, a.z * b.x - a.x * b.z, a.x * b.y - a.y * b.x);}
 inline Point cross(Point o, Point a, Point b) { return cross(a - o, b - o);}
 inline double veclen(Point x) { return sqrt(dot(x, x));}
 inline double area(Point o, Point a, Point b) { return veclen(cross(o, a, b)) / 2.0;}
 inline double volume(Point o, Point a, Point b, Point c) { return dot(cross(o, a, b), c - o) / 6.0;}

 struct _3DCH{
     Point pt[N];
     int pcnt;
     struct Face {
         int a, b, c;
         bool ok;
         Face() {}
         Face(int a, int b, int c) : a(a), b(b), c(c) { ok = true;}
     } fc[N << ];
     int fcnt;
     int fid[N][N];

     bool outside(int p, int a, int b, int c) { return sgn(volume(pt[a], pt[b], pt[c], pt[p])) < ;}
     bool outside(int p, int f) { return outside(p, fc[f].a, fc[f].b, fc[f].c);}
     void addface(int a, int b, int c, int d) {
         if (outside(d, a, b, c)) fc[fcnt] = Face(c, b, a), fid[c][b] = fid[b][a] = fid[a][c] = fcnt++;
         else fc[fcnt] = Face(a, b, c), fid[a][b] = fid[b][c] = fid[c][a] = fcnt++;
     }

     bool dfs(int p, int f) {
         if (!fc[f].ok) return true;
         int a = fc[f].a, b = fc[f].b, c = fc[f].c;
         if (outside(p, a, b, c)) {
             fc[f].ok = false;
             if (!dfs(p, fid[b][a])) addface(p, a, b, c);
             if (!dfs(p, fid[c][b])) addface(p, b, c, a);
             if (!dfs(p, fid[a][c])) addface(p, c, a, b);
             return true;
         }
         return false;
     }
     void build() {
         bool ok;
         fcnt = ;
         sort(pt, pt + pcnt);
         pcnt = unique(pt, pt + pcnt) - pt;
         ok = false;
         for (int i = ; i < pcnt; i++) {
             if (sgn(area(pt[], pt[], pt[i]))) {
                 ok = true;
                 swap(pt[i], pt[]);
                 break;
             }
         }
         if (!ok) return ;
         ok = false;
         for (int i = ; i < pcnt; i++) {
             if (sgn(volume(pt[], pt[], pt[], pt[i]))) {
                 ok = true;
                 swap(pt[i], pt[]);
                 break;
             }
         }
         if (!ok) return ;
         addface(, , , );
         addface(, , , );
         addface(, , , );
         addface(, , , );
         for (int i = ; i < pcnt; i++) {
             for (int j = fcnt - ; j >= ; j--) {
                 if (outside(i, j)) {
                     dfs(i, j);
                     break;
                 }
             }
         }
         int sz = fcnt;
         fcnt = ;
         for (int i = ; i < sz; i++) if (fc[i].ok) fc[fcnt++] = fc[i];
     }
     double facearea() {
         double ret = 0.0;
         for (int i = ; i < fcnt; i++) ret += area(pt[fc[i].a], pt[fc[i].b], pt[fc[i].c]);
         return ret;
     }
     bool same(int i, int j) {
         int a = fc[i].a, b = fc[i].b, c = fc[i].c;
         if (sgn(volume(pt[a], pt[b], pt[c], pt[fc[j].a]))) return false;
         if (sgn(volume(pt[a], pt[b], pt[c], pt[fc[j].b]))) return false;
         if (sgn(volume(pt[a], pt[b], pt[c], pt[fc[j].c]))) return false;
         return true;
     }
     int facecnt() {
         int cnt = ;
         for (int i = ; i < fcnt; i++) {
             bool ok = true;
             for (int j = ; j < i; j++) {
                 if (same(i, j)) {
                     ok = false;
                     break;
                 }
             }
             if (ok) cnt++;
         }
         return cnt;
     }
 } ch;

 int main() {
 //    freopen("in", "r", stdin);
     int n;
     double x, y, z;
     while (~scanf("%d", &ch.pcnt)) {
         for (int i = ; i < ch.pcnt; i++) {
             scanf("%lf%lf%lf", &x, &y, &z);
             ch.pt[i] = Point(x, y, z);
         }
         ch.build();
 //        printf("%.3f\n", ch.facearea());
         printf("%d\n", ch.facecnt());
     }
     return ;
 }

　　三维凸包还是挺简单的，不用抄模板也能敲出来。不过，还不熟练，敲了接近一个钟。

——written by Lyon