HDU 1392 Surround the Trees（几何 凸包模板）

 1 #include<cmath>
 2 #include<cstdio>
 3 #include<algorithm>
 4 using namespace std;
 5 
 6 #define MAXN 100+10
 7 
 8 struct Point{
 9     double x, y;
10 
11     Point(double x = 0, double y = 0): x(x), y(y){}
12 
13     void scan(){
14         scanf("%lf %lf", &x, &y);
15     }
16 };
17 
18 typedef Point Vector;
19 
20 Vector operator - (Vector A, Vector B){
21     return Vector(A.x - B.x, A.y - B.y);
22 }
23 
24 double dot(Vector A, Vector B){//点乘
25     return A.x * B.x + A.y * B.y;
26 }
27 double length(Vector A){//向量模
28     return sqrt(dot(A, A));
29 }
30 double cross(Vector A, Vector B){//叉乘
31     return A.x * B.y - A.y * B.x;
32 }
33 
34 int n;
35 Point p[MAXN], stack[MAXN];
36 
37 bool input(){
38     scanf("%d", &n);
39     if(n == 0) return false;
40     for(int i = 0; i < n; ++i){
41         p[i].scan();
42     }
43     return true;
44 }
45 
46 bool cmp(Point A, Point B){
47     return A.x < B.x || (A.x == B.x && A.y < B.y);
48 }
49 
50 int convex_hull(){//Adrew算法 求凸包
51     sort(p, p + n, cmp);
52 
53     int m = 0;
54     for(int i = 0; i < n; ++i){
55         while(m > 1 && cross(stack[m - 1] - stack[m - 2], p[i] - stack[m - 2]) <= 0) --m;
56         stack[m ++] = p[i];
57     }
58     int k = m;
59     for(int i = n - 1; i >= 0; --i){
60         while(m > k && cross(stack[m - 1] - stack[m - 2], p[i] - stack[m - 2]) <= 0) --m;
61         stack[m ++] = p[i];
62     }
63     if(n > 1) --m;
64     return m;
65 }
66 
67 void solve(){
68     n = convex_hull();
69     if(n == 1){//当只有一个点时，绳子长度为0
70         puts("0.00");
71         return ;
72     }
73     if(n == 2){//当只有两个点时，绳子长度为两点之间的长度
74         printf("%.2lf\n", length(stack[0] - stack[1]));
75         return ;
76     }
77     double ans = 0;//绳子的累加长度
78     stack[n] = stack[0];//为了算第n点到第1点的距离
79     for(int i = 0; i < n; ++i){
80         ans += length(stack[i] - stack[i + 1]);
81     }
82     printf("%.2lf\n", ans);
83 }
84 
85 int main(){
86     while(input()){
87         solve();
88     }
89     return 0;
90 }