[HDU1392]Surround the Trees

 1 #include<cmath>
 2 #include<cstdio>
 3 #include<cctype>
 4 #include<algorithm>
 5 inline int getint() {
 6     char ch;
 7     while(!isdigit(ch=getchar()));
 8     int x=ch^‘0‘;
 9     while(isdigit(ch=getchar())) x=(((x<<2)+x)<<1)+(ch^‘0‘);
10     return x;
11 }
12 struct Point {
13     int x,y;
14     Point operator - (const Point &x) const {
15         return (Point){this->x-x.x,this->y-x.y};
16     }
17     int operator * (const Point &x) const {
18         return this->x*x.y-x.x*this->y;
19     }
20 };
21 int dist2(const Point x,const Point y) {
22     return (x.x-y.x)*(x.x-y.x)+(x.y-y.y)*(x.y-y.y);
23 }
24 double dist(const Point x,const Point y) {
25     return sqrt(dist2(x,y));
26 }
27 const int N=101;
28 Point p[N];
29 bool operator < (const Point &p1,const Point &p2) {
30     int s=(p1-p[0])*(p2-p[0]);
31     return s<0||(!s&&(dist2(p1,p[0]))>=dist2(p2,p[0]));
32 }
33 bool judgeOnLeft(int p0,int p1,int p2) {
34     double s=(p[p1]-p[p0])*(p[p2]-p[p0]);
35     return s<0||(!s&&(dist2(p[p1],p[p0]))>=dist2(p[p2],p[p0]));
36 }
37 inline void swap(Point &a,Point &b) {
38     Point t;
39     t=a;
40     a=b;
41     b=t;
42 }
43 int main() {
44     int n;
45     while(n=getint()) {
46         if(n==1) {
47             puts("0.00");
48             continue;
49         }
50         if(n==2) {
51             Point a,b;
52             a.x=getint(),a.y=getint();
53             b.x=getint(),b.y=getint();
54             printf("%.2f\n",dist(a,b));
55             continue;
56         }
57         int k=0;
58         for(int i=0;i<n;i++) {
59             p[i].x=getint(),p[i].y=getint();
60             if((p[i].x<p[k].x)||((p[i].x==p[k].x)&&(p[i].y<p[k].y))) k=i;
61         }
62         swap(p[0],p[k]);
63         std::sort(&p[1],&p[n]);
64         p[n]=p[0];
65         int s[N],top=1;
66         s[0]=0;
67         s[1]=1;
68         for(int i=2;i<=n;i++) {
69             while(top&&judgeOnLeft(s[top-1],i,s[top])) top--;
70             s[++top]=i;
71         }
72         s[top]=s[0];
73         double ans=0;
74         for(int i=1;i<=top;i++) {
75             ans+=dist(p[s[i]],p[s[i-1]]);
76         }
77         printf("%.2f\n",ans);
78     }
79     return 0;
80 }