【旋转卡壳】POJ 2079 Triangle

 1 #include <iostream>
 2 #include <vector>
 3 #include <algorithm>
 4 #include <cmath>
 5 using namespace std;
 6  
 7 const int MAX_N = 100007;
 8  
 9 struct Point {
10     double x, y;
11     Point() {}
12     Point(double x, double y) : x(x), y(y) {}
13     Point operator + (const Point& p){ return Point(x + p.x, y + p.y); }
14     Point operator - (const Point& p){ return Point(x - p.x, y - p.y); }
15     Point operator * (const double& d){ return Point(x * d, y * d); }
16     bool operator < (const Point& a) const {
17         if (x != a.x) return x < a.x;
18         else return y < a.y;
19     }
20 };
21  
22 int n, top;
23 Point p[MAX_N], st[MAX_N];
24 
25 double cross(Point p0, Point p1, Point p2) {
26     return (p1.x - p0.x) * (p2.y - p0.y) - (p2.x - p0.x) * (p1.y - p0.y);
27 }
28 
29 void covexHull() {
30     sort(p, p + n); 
31     st[0] = p[0];
32     st[1] = p[1];
33     top = 1;
34     for(int i = 2; i < n; ++i) {
35         while(top > 0 && cross(st[top], p[i], st[top - 1]) <= 0)
36             top--;
37         st[++top] = p[i];
38     }
39     int temp = top;
40     for(int i = n - 2; i >= 0; --i) {
41         while(top > temp && cross(st[top], p[i], st[top - 1]) <= 0)
42             top--;
43         st[++top] = p[i];
44     }
45 }
46 
47 double A(Point a, Point b, Point c) {
48     return fabs(cross(a, b, c)) / 2;
49 }
50 
51 double cal() {
52     covexHull();
53     int p = 0, q = 1, r = 2;
54     double area = A(st[p], st[q], st[r]);
55     while(true) {
56         int pp = p, qq = q, rr = r;
57         while(fabs(cross(st[(r + 1) % n], st[q], st[p])) > fabs(cross(st[r], st[q], st[p]))) {
58             area = max(area, A(st[(r + 1) % n], st[q], st[p]));
59             r = (r + 1) % n;
60         }
61         while(fabs(cross(st[r], st[(q + 1) % n], st[p])) > fabs(cross(st[r], st[q], st[p]))) {
62             area = max(area, A(st[r], st[(q + 1) % n], st[p]));
63             q = (q + 1) % n; 
64         }
65         while(fabs(cross(st[r], st[q], st[(p + 1) % n])) > fabs(cross(st[r], st[q], st[p]))) {
66             area  =max(area, A(st[r], st[q], st[(p + 1) % n]));
67             p = (p + 1) % n;
68         }
69         if (pp == p && qq == q && rr == r) r = (r + 1) % n;
70         if(r == 0) break;
71     }
72     return area;
73 }
74 
75 int main() {
76     while (1 == scanf("%d", &n)) {
77         if (-1 == n) break;
78         for (int i = 0; i < n; ++i)
79             scanf("%lf%lf", &p[i].x, &p[i].y);
80         double ans = cal();
81         printf("%.2f\n", ans);
82     }
83     return 0;
84 }